Just when we think NFL players have more in the tank, some walk away while on the rise, in their prime years or after an impressive season.

Last August, former Indianapolis Colts quarterback Andrew Luck abruptly retired, which sent shock waves throughout the NFL landscape. He joined a growing list of names to leave the game before showing signs of a significant decline on the field.

We'll remember some of the most surprising retirements over the past 30 years and delve into why those players chose to leave the game.

All of the selections below retired in their early 30s or younger as ascending talents or at the peak of their careers. We've excluded players who made a comeback on an active roster, such as Rob Gronkowski, whom the Tampa Bay Buccaneers acquired from the New England Patriots in April.

QB Andrew Luck, Indianapolis Colts (2012-18)

Retirement Age: 29

During the fourth quarter of the Indianapolis Colts' third preseason game against the Chicago Bears, fans caught wind of the surprising news. The team's franchise quarterback, Andrew Luck, planned to retire before the 2019 season.

Following a strong finish to the 2018 campaign, which ended in an AFC divisional-round loss to the Kansas City Chiefs, the Colts had a promising outlook for the following term.

In 2018, Luck played a full 16-game slate and went to the Pro Bowl for the first time since 2014. More importantly, he only took 18 sacks behind an offensive line that failed to protect him early in his career.

Unfortunately, Luck took too many hits between 2012 and 2016. He missed the entire 2017 campaign because of a shoulder injury. Despite his efforts to push forward, the wear and tear on his body forced him out of the game. Full of emotion, he addressed the media last August:

"This is not an easy decision. Honestly it's the hardest decision of my life. But it is the right decision for me. For the last four years or so I've been in this cycle of injury, pain, rehab; injury, pain, rehab. And it's been unceasing and unrelenting both in-season and offseason. I felt stuck in it. The only way I see out is to no longer play football."

Luck went to four Pro Bowls, led the league in touchdowns (40) for the 2014 term and earned the 2018 Comeback Player of the Year award, throwing for 4,593 yards, 39 touchdowns and 15 interceptions.

RB Barry Sanders, Detroit Lions (1989-98)

Retirement Age: 31

Known for his quick cuts and ability to make defenders miss, Barry Sanders' run style resembles poetry in motion. He broke tackles, shifted his body, changed direction and cranked up the speed in the open field. The Hall of Fame running back remains arguably the most elusive ball-carrier to play the game.

Sanders won four rushing titles and ran for at least 1,115 yards in each of his 10 seasons. After 1998, he noticed his attitude toward the game had changed over the years. He talked about it in his book Barry Sanders: Now You See Him...His Story in His Own Words (h/t the Associated Press, via ESPN).

"I had already achieved a level of success that gave me much satisfaction and pride. I didn't need to pass Walter [Payton] to prove that to myself," Sanders wrote. He also said: "After all these years, I'd come full circle. It was tough to stay focused and motivated."

Sanders didn't miss a game from 1994 to 1998. Despite the hits running backs take, he had been in good shape, but he walked away with his head held high with two Offensive Player of the Year awards, a league MVP (1997), 10 Pro Bowl nods and six All-Pro seasons.

RB Tiki Barber, New York Giants (1997-2006)

Retirement Age: 31

Tiki Barber didn't become a featured running back right away. As a second-round pick out of Virginia, he took on a reserve role in the New York Giants backfield for three seasons.

In his fourth term, Barber ran for 1,006 yards and eight touchdowns and caught 70 passes for 719 yards and a score as the lead running back over Ron Dayne. He had glaring ball-security issues, fumbling 35 times between the 2000 and 2003 seasons.

As a player, Barber aged well and did a better job of holding on to the football later in his career, fumbling nine times from 2004 through 2006. He went to the Pro Bowl in each of those seasons and had an All-Pro showing in 2005.

Although Barber played at a high level on the other side of 30, he experienced a moment that steered him toward retirement in his final campaign.

"To put it into clarity," he said on the Talk of Fame Network, "my first game against Philadelphia [in] my last season we played at Philly. It was at the new 'Linc' (Lincoln Financial Field), I got the crap beat out of me by Jeremiah Trotter, and I walked out that game saying, 'I'm done.'"

Barber also said he had "many more interests" outside of football.

However, he filed paperwork for a return in 2011. He worked out for the Miami Dolphins but didn't sign with a team. At 35 years old, the ball-carrier allowed too much time to pass before attempting a comeback.

After facing a cold market, the 10-year NFL veteran returned to his post-retirement media career and currently co-hosts Tiki and Tierney on CBS Sports Radio.

RB Robert Smith, Minnesota Vikings (1993-2000)

Retirement Age: 28

Robert Smith battled injuries through the first four years of his career, missing 23 games before he took control of the starting position with the Minnesota Vikings.

In 1997, Smith broke out for 1,266 yards and six touchdowns. He went to his first Pro Bowl after the following term. The Vikings ball-carrier went out at his best, rushing for a career-high 1,521 yards and seven touchdowns in 2000.

Smith's decision to end his career weighed on his mind for years, per Mark Podolski of the News-Herald.

"I had been thinking about retirement for a while," he said. "Football is such a tough sport on a player's body. I kept thinking, 'What's my body going to be like years from now?'"

As a player who dealt with constant injuries coming out of college, Smith took a long-term view of his life beyond football. He thought about a return with the Arizona Cardinals in 2004 to rejoin his former head coach Dennis Green but chose to continue his life away from the field.

WR Calvin Johnson, Detroit Lions (2007-15)

Retirement Age: 30

Few players had Calvin Johnson's physical stature and ability. At 6'5", 239 pounds, he ran a 4.35-second 40-yard dash at the NFL Scouting Combine.

Johnson didn't just look the part—he dominated defensive backs on game day. The former Detroit Lions wideout has the most receiving yards in a single season (1,964). He's also tied with Minnesota Vikings wideout Adam Thielen for most consecutive games (eight) with 100-plus receiving yards.

Amid six straight campaigns with 1,077 receiving yards or more, Johnson led the league in the category for back-to-back terms (2011-12) and didn't show much sign of slowing down before he retired in 2016. In his final season, he ranked 10th in receiving yards with nine touchdowns.

Through most of his career, the Lions struggled to win games, even going 0-16 in 2008. Yet Detroit could always count on its top receiver. Johnson tied Larry Fitzgerald for the most touchdown grabs (12) during his winless campaign.

Johnson told reporters in 2017 that the Lions' inability to contend for a title factored into his decision to hang up his cleats:

"I was stuck in my contract with Detroit, and they told me, they would not release my contract, so I would have to come back to them. I didn't see the chance for them to win a Super Bowl at the time, and for the work I was putting in, it wasn't worth my time to keep on beating my head against the wall and not going anywhere."

The Lions made two playoff appearances with Johnson, and they haven't won a postseason game since 1992. Other than retiring, he didn't see a way out of a losing culture.

DE Aaron Schobel, Buffalo Bills (2001-09)

Retirement Age: 32

The Buffalo Bills had a reliable pass-rusher in Aaron Schobel for about a nine-year stretch. During his time with the club, he led the team in sacks (78) by a wide margin.

Schobel went to two Pro Bowls and logged four campaigns of double-digit sacks. He played eight full seasons, but a foot injury cost him 11 games in 2008. The TCU product bounced back in his final term, registering 10 sacks and 17 tackles for loss, but he told the Associated Press when he retired in 2010 that he didn't have the drive to play the game anymore (via ESPN):

"I really didn't have a lot of fun playing last year. The passion wasn't there, so it was time. I took as much time as I felt like I needed to make the decision, and I think I made the right one. ... The decision came down to whether I wanted to play. I didn't really feel like I wanted to do it. A lot of people probably think I'm crazy, but whenever you feel like it's time, it's time."

While the Bills released Schobel before the 2010 season, he could've landed with another team because of his ability to pressure the pocket. He likely passed up millions on a new deal, but his desire to play had run its course.

LB Luke Kuechly, Carolina Panthers (2012-19)

Retirement Age: 28 

Luke Kuechly ended his pro journey in January. He has since accepted a pro scout position with the Carolina Panthers—the team that selected him in the first round of the 2012 draft.

When you speak about second-level defenders with range and versatility, Kuechly fits the description. He came into the league with the skill set to match the new wave of athletic off-ball linebackers who weigh closer to 230 pounds rather than 240.

Kuechly earned Defensive Rookie of the Year honors and then won Defensive Player of the Year in 2013. He went to seven Pro Bowls and had five All-Pro seasons. Aside from being a tackle machine, he could attack the backfield (75 tackles for loss) and drop back in coverage (66 pass breakups and 18 interceptions).

At the end of the 2019 campaign, Kuechly felt he put his best foot forward for eight seasons and didn't have much left

"I just knew that, you know, I had given everything I had, to be honest with you, I was done at that point," he told NBC Sports Network (h/t Alaina Getzenberg of the Charlotte Observer). "It had nothing to do with changes in coaches or anything like that. I just think I knew in my heart that I gave everything I had and, at this point, it was time to move in a different direction."

Kuechly suffered "serious" concussions during his career, one of which caused him to miss six contests in 2016. He only sat out one game over the following three seasons. He'll move into the Panthers front office relatively healthy.

LB Patrick Willis, San Francisco 49ers (2007-14)

Retirement Age: 30

Patrick Willis provided a physical presence in the middle of the San Francisco 49ers defense. He patrolled the field sideline to sideline but also came down near the line of scrimmage to pressure the pocket.

Willis led the league in tackles twice, once as a rookie, and recorded 20.5 sacks and 60 tackles for loss for his career, which is impressive for a second-level defender. He had the anticipation and quickness to blow up plays in the backfield.

Willis earned Defensive Rookie of the Year honors in 2007 and never looked back, going to seven Pro Bowls with five All-Pro seasons within eight years. Although a toe injury sidelined him for 10 games in 2014, his decision to retire went beyond a foot ailment. He spoke in depth about a difficult conclusion:

"As I stand up here today, it's tough, it's hard, but it's also easy at the same time. I knew there would be a day I'd leave, and I always told myself that I wanted it to be on my terms. ... Honestly, I pay attention to guys when they're finished playing, walking around like they've got no hips and they can't play with their kids. They can barely walk."

Willis may have returned to Pro Bowl or All-Pro form after missing a majority of the 2014 campaign, but he chose to walk away rather than endure further wear and tear.

LB Chris Borland, San Francisco 49ers (2014)

Retirement Age: 24

You might have missed Chris Borland's time in the NFL if you paid little attention to the 2014 season. He's the biggest "what if" retiree in recent memory.

With Patrick Willis on the sideline for more than half of the 2014 term, Borland filled a void at linebacker for the 49ers, registering 108 tackles, 12 for loss, four pass breakups and two interceptions.

Like Willis, Borland took a preemptive approach to safeguard his well-being, per ESPN's Mark Fainaru-Wada and Steve Fainaru:

"I feel largely the same, as sharp as I've ever been. For me, it's wanting to be proactive. I'm concerned that if you wait 'til you have symptoms, it's too late. ... There are a lot of unknowns. I can't claim that X will happen. I just want to live a long, healthy life, and I don't want to have any neurological diseases or die younger than I would otherwise."

At the time Borland retired, studies that linked head trauma to brain disease raised serious discussions about player safety and life after football. Despite the Wisconsin product's strong career start, he didn't want to put his health at long-term risk.

Borland quickly rose through the ranks and went out in a flash.

OLB Jason Worilds, Pittsburgh Steelers (2010-14)

Retirement Age: 27

Over the course of five seasons, Jason Worilds worked his way into a starting role within the Pittsburgh Steelers defense. He made a significant jump from 2012 to 2013, transitioning from a backup role to the first unit.

Once Worilds saw an uptick in defensive snaps, he made a bigger impact on the pass rush, logging 15.5 sacks combined in the 2013 and 2014 campaigns. He may have commanded a lucrative salary on the free-agent market but went in a different direction.

Ed Bouchette reported for the Pittsburgh Post-Gazette that Worilds chose to follow a calling in faith.

"Worilds, 27, abruptly announced his retirement early Wednesday morning on Twitter, and Steelers sources say they were told it is to devote his time to working for his religion, the Jehovah's Witnesses," Bouchette wrote.

Worilds released a statement through his then-agent Jason Bernstein, but he didn't go into specifics on the factors that led to an abrupt career end.

Steelers general manager Kevin Colbert described Worilds as an "ascending player" before his retirement but was prepared to allow him to test free agency in 2015, per Scott Brown of ESPN. The outside linebacker also likely left millions on the table.