In the modern NFL, quarterbacks are playing into their 40s or close to it. Tom Brady is 44. Drew Brees played until he was 41. Peyton Manning retired at 39. Philip Rivers and Eli Manning hung up their cleats at 39. Ben Roethlisberger is 39. Aaron Rodgers is 37. 

So it's hard to blame Las Vegas Raiders quarterback Derek Carr, who is 30, for believing he can go until he's 45. 

"I feel like I can play another 15 years, without a doubt," he told reporters Wednesday. "Nothing's hurting. It's the middle of camp, and I feel great. I feel strong, I feel fast, I feel explosive. And I think it's just the way we train now."

Carr, who turned 30 in March, believes advancements in areas like nutrition and training will help modern players extend their careers: 

"The only thing that's different is I've got gray hairs in my beard. I haven't grown it out in a while; I've got five gray hairs. What is going on? I feel like I'm 20. I honestly believe this—that our generation, the way we can take care of our bodies with football, I think 30 is the new 20. ... That comes from science, and that comes with more understanding."

NFL rules better protecting quarterbacks may play a role as well. But it's clear that the position has enjoyed a lot of longevity in the past 20 years, at least among the top quarterbacks in the league. It remains to be seen if Carr will be considered part of that group.