Not sure.
Until 20, they're kids.
At 20, they leave the baby fat behind (and from a teenager's perspective, they're not really "young" anymore), but they're still very young, they learn and they learn quick.
At 22, the post-teenage age is over too, and if you're a pro-athlete, you'd better be getting your act together soon. Still young though.
23-25, strictly physically, you're at your best. That's where speed and explosiveness meet your peak or near-peak strength. Still young, but in a mature way.
26-30, not really young anymore, but still far from old. Regeneration starts slowing down and so do you. Meanwhile, years, then months, then fortnights, weeks, eventually even days and hours run faster. Quality sleep is harder to get. On the other hand, your strength peaks and you know way more than all those speedy flashy rookies. You're in the "had I known all this ten years ago, boy, I would have conquered the..." blah blah phase. Possibly your best.
Past 30, it all goes down. Not overnight, but at 32/33 at the latest, you know the best years are behind, and it keeps getting worse by a year.
Over 35, you find yourself having barely enough time to hang on, let alone to learn or improve. This is the true survival phase. The average life expectancy of cavemen was 25. Based on what it took to survive in the old stone age, you're well past due, and let's face it, any pro sport is a jungle.
Past 40, you're either retired, or an aberration.