There's no question that playing in the NFL is physically taxing, but there isn't a clear-cut answer how players are thriving well into their 30s. Even head coach Aaron Glenn, who played until he was 36, doesn't have the secret.

"God given, and it's not for everybody," Glenn said. "You have to give [Demario Davis, 37] a lot of credit and all these guys, Harrison Phillips [30], Geno Smith [35], all those guys that can play into their mid-to-upper 30s, the way that they take care of their body and how serious they are about their game and making sure they're there and they're accountable for being on the field for their teammates."

Staying sharp is what Davis prides himself in. Outside of his rookie season, Davis has played at least 70% of defensive snaps each year — now, he is preparing for his 15th NFL season, and third stint with the Jets.

"I think that's kind of what I build my career on now is, you know, how's this guy 37 and still looking like he's 27, 28," Davis said earlier this offseason. "I have a phenomenal team that's working on me year-round. It's almost like a car once it comes off the track: it's always in the garage getting worked on and that has helped exponentially."

So, what's the key to longevity? Davis believes part of it is because the game has shifted more toward "speed and agility" rather than just "hard knocks."

"I think the science, just in terms of recovery, plays a major part, and I try to maximize on all types of technology that's out there and so I think all of that lends itself to longer careers and I don't think that I'm the exception," Davis said. "I think I'm the new norm to players who take care of their body, maximize the game and all this around it and are able to stay relatively healthy, they're going to have careers very similar to mine."

Currently, there are seven Jets that are at least 30 years old — Davis is the oldest. Phillips just turned 30 this year and looks to his fellow defensive lineman as a role model. Last season, the Jets had one of the youngest rosters in the NFL.

"I'm younger than he is and so if I'm waking up like, 'Ah' and he's out there, I'm like all right I don't really have much ground to stand on, this guy's played a lot more snaps in this league and is still out there doing the same things," Phillips said.

Lasting in the NFL from a production standpoint also isn't easy. Position turnover can be common, and at times expected. Davis credits Smith, who is 2 years younger than him, for always showing up "with the right attitude."

"This is high-level business," Davis said. "To play past 3 years, 8 years, a decade in this business is very tough, just ask guys who've done it, and to do it at the most important position says a lot. To do it as a starter says even more. So you would have at this point what you would believe to have is the reason he keeps ending up on rosters is the reason he keeps ending up on the starting lineup, so I'm excited to have him at this point in his career."

Some habits never die and for Smith, he's never lost his love of football or desire to study just as hard as he did when he was a rookie in 2013.

"I've just gotten older," Smith said. "Overall, I've always felt like I've had talent, but I feel like my fundamentals have grown and that's on and off the field. I feel like the leadership aspect has grown throughout my experience in the league and all the great quarterbacks I've been around, great coordinators as well as I'm a lot more calm. It's a lot less of what am I trying to figure out, what to expect, I kind of know what to expect now and so just a lot more calm in my demeanor."