Young champions, such as Jon Jones and Jose Aldo, are becoming the norm in the world's top MMA promotion.

There was a time when a 45-year-old Randy Couture reigned over the UFC's heavyweight division. Nowadays, 31-year-old Cain Velasquez is the eldest of the organization's eight current titleholders, and he's the only UFC champion to have seen his 30th birthday.

A new breed of mixed martial artists is replacing the old guard. 

They are going to be underdogs on The Ultimate Fighter 20, but Rose Namajunas and Paige VanZant will both have opportunities to become the youngest champion in UFC history should they win the competition and become the inaugural women's strawweight champion.

Before those ladies step into the Octagon, here are the 10 youngest UFC champions.

10. Tito Ortiz

Aged: 25 years, 2 months (9,213 days)

Tito Ortiz was 22 years old when he made his MMA debut at UFC 13. He lost to Guy Mezger as a replacement in the light heavyweight tournament final.

Only three fights later and after avenging his loss to Mezger, "The Huntington Beach Bad Boy" was awarded a shot at Frank Shamrock, another young UFC champion.

Ortiz was stopped in the fourth round of that matchup, which would have made him the fourth youngest UFC titleholder had he won. 

Shamrock would retire following that win, and Ortiz was matched up against Wanderlei Silva in a bout to determine the new light heavyweight champion. After beating Silva in a decision, Ortiz carried the UFC through some tough times as the brash king of the 205-pound class.

9. Jens Pulver

Aged: 25 years, 2 months (9,211 days)

Jens Pulver made his MMA debut at 23 years old. Within two years, he amassed 12 fights and was booked to meet Caol Uno in a matchup that would determine the inaugural UFC lightweight champion.

"Lil' Evil" defeated Uno in a majority decision to capture the lightweight crown. He then went on to defend his belt against Dennis Hallman and BJ Penn before vacating the title due to a contract dispute—a move that ultimately wasn't the best decision for his legacy.

Still, being the original UFC lightweight titleholder is something Pulver can never have taken away from him.

8. BJ Penn

Aged: 25 years, 1 month (9,180 days)

BJ Penn's MMA career commenced at 22 years old, and he was almost immediately considered among the best in the lightweight division.

Only four fights in, Penn met Jens Pulver and had a shot at the 155-pound strap. "The Prodigy" fell short in a majority decision. Had he won that bout, Penn would be the youngest champion in UFC history at 23. 

Just three fights later, Penn found himself battling Caol Uno for the belt Pulver vacated. However, he was again unable to claim the championship, as he fought to a draw. Winning that contest would have made Penn the third youngest champion inside the Octagon.

Finally, 11 months after his loss to Uno, Penn was able to capture a UFC championship by submitting welterweight titleholder Matt Hughes. 

7. Ricco Rodriguez

Aged: 25 years, 1 month (9,170 days)

Beginning his MMA career at 21 years old, Ricco Rodriguez made his UFC debut two years later after competing with Pride FC and other smaller organizations.

After winning his first four UFC bouts, Rodriguez was paired with Randy Couture in a matchup to determine a new heavyweight champion. Two months earlier, Josh Barnett was stripped of the belt for failing his drug test at UFC 36, where he had dethroned Couture.

Rodriguez ended up stopping Couture with elbows in the fifth round. Although he ended up losing his next three fights and would never return to the Octagon, Rodriguez earned his place in UFC history, edging BJ Penn by only 10 days as the seventh youngest champion the promotion has seen.

6. Frank Mir

Aged: 25 years, 0 months (9,158 days)

Frank Mir had two MMA bouts before debuting in the UFC as a 22-year-old. 

After winning five of his first six fights inside the Octagon, Mir was awarded a shot at heavyweight champion Tim Sylvia. Less than one minute in, Mir snapped the champion's arm to become one of the youngest champions in UFC history.

Unfortunately, a motorcycle accident prevented Mir from taking full advantage of his early success. Before he was able to defend his undisputed title, the young champion was stripped of the belt due to inactivity while recovering from his significant injuries.

5. Frank Shamrock

Aged: 25 years, 0 months (9,144 days)

Frank Shamrock accumulated 22 MMA fights before first appearing inside the Octagon in December 1997.

As he was so experienced outside the UFC, Shamrock's debut with the organization was a fight with Kevin Jackson to determine the inaugural light heavyweight champion.

Only two weeks younger than Frank Mir when he submitted Tim Sylvia, Shamrock also claimed a UFC belt with an armbar in less than 60 seconds. 

Shamrock defended his championship four times before taking a break from MMA. Had he continued his reign, Shamrock had the potential to go down as one of the more dominant champions in UFC history.

Still, Tito Ortiz and Jon Jones are the only fighters to earn more consecutive UFC light heavyweight title defenses. 

4. Carlos Newton

Aged: 24 years, 8 months (9,026 days)

Carlos Newton began competing in MMA at only 19 years old. 

He competed at UFC 17 and with various other promotions before returning to the Octagon for a title shot at UFC 31. With a rare bulldog choke, Newton dethroned Pat Miletich and claimed the welterweight championship.

Six months later, Newton was beaten in his first title defense against Matt Hughes. Although he had Hughes in a tight triangle choke, Newton was slammed and knocked out by an arguably already unconscious adversary. 

3. Josh Barnett

Aged: 24 years, 4 months (8,898 days)

Like Carlos Newton, Josh Barnett began competing in MMA when he was 19 years old. 

He made his UFC debut a week after his 23rd birthday and won three of his first four fights inside the Octagon. Despite failing his drug test at UFC 34, Barnett was awarded a title shot in his following bout against Randy Couture, who he defeated at only 24 years old.

Barnett was later found to have failed his drug test for that bout as well. While that would have negated his win in today's world, Barnett kept his victory over Couture and was only stripped of his heavyweight championship.

Despite the controversial nature of his champion status, Barnett held the distinction of youngest UFC champion ever for more than eight years. 

2. Jose Aldo

Aged: 24 years, 2 months (8,838 days)

Jose Aldo began fighting in Brazil before turning 18 years old. By the time he was 21 years old, Aldo had signed with WEC, which was then the premier organization for featherweights.

By knocking out his first five WEC opponents, Aldo was given a shot at 145-pound champion Mike Brown. The Brazilian also stopped Brown to become the WEC featherweight champion at 23 years old.

Had that fight occurred inside the Octagon, Aldo would be the youngest champion in UFC history.

Instead, Aldo had to win two more fights while waiting 12 months to be promoted to UFC champion. For a brief period, that promotion did make Aldo the youngest fighter to hold a UFC belt, but the distinction did not last long. 

1. Jon Jones

Aged: 23 years, 8 months (8,644 days)

Jon Jones got started in MMA a little later than some of the other fighters near the top of this list, but he made astronomic leaps and bounds toward the top of the light heavyweight division.

Beginning his career a few months before his 21st birthday, Jones needed only six fights to prove himself as a valuable prospect to UFC officials. After signing with the promotion, it took Jones only six more fights to show he was deserving of a shot at the 205-pound championship.

At 23 years old, Jones stopped Mauricio Rua to become the youngest champion in UFC history. Nearly three years later, Jones is still the light heavyweight titleholder and is establishing himself as one of the best fighters to ever step into the Octagon.