Youth ends after 25 for men. That is the age our bodies are 100% done developing and after that we just decline and slowly lose virility. A little bit earlier for women but not by much (the following numbers apply to men)
I've done a lot of research on aging and peak physical age and this is what I've found (I think physical and mental performance is a good measure of "youth")
Peak age for explosive energy and speed (namely sprinting and tennis): 22-24
Peak age for Crossfit performance in males: 23
Peak age for cognition and reflexes: 24
Peak age for untrained muscle mass: 25
Peak DHEA: 25
Peak testosterone: 25
Peak bone mass: 25-30
Age that the male brain is finished developing: 25 (female brains develop at around 22)
The male human body seems to be at its most virile at about age 22-25. Keep in mind that the reason other athletes like football and basketball players peak in their late 20s is because those sports rely a lot on skill and knowledge of the game, and not only raw athleticism. Also if you just go around researching what age most women find physically attractive in men, most will give either 25-30 or 20-25 as the prime ages.
Your body is downhill after 25. It's true that you can build muscle and endurance into your 30s and 40s but you have the most potential for gains in your early 20s.
I'll leave this graph here, too. It demonstrates how mortality rate changes with age, which is a good way of measuring youth and aging. I speculate that the sharp increase in mortality in the teens is due to typical risky teenage behavior. It levels out in the early 20s and only slightly increases in the late 20s for women. After 30 aging is full force.
The moment in which my adulthood began, with no going back, happened when I was 22. But I stopped feeling young this year (25).
My skin and hair are fine. But the agepill has taken so much from me mentally.