Physically I think youth ends in the early 30s (latest), maybe a couple of years later for men. I'm 34 now, and I've noticed small things like hair being a bit more dull, preferring to wear flat shoes, overall concerning myself more with skincare than make-up, preferring to come home earlier (real early like 8pm) instead of staying out late, etc. We all age, and when we notice ourselves going "downhill" in small ways, I believe our youth physically is over.
Mentally, I believe you can be youthful at any age, but we have to be aware that we can easily lose our mental youth. I would equate mental youth with childlike wonder, with a thirst for knowledge and ability to think outside the box. A youthful elderly person may still like to go on a merry-go-round, however if he does not enjoy any ride at all, I would say his youth is over. An older person can still have big dreams (pursuing new hobbies, travel, etc.) and can be considered youthful, and find pleasure in the little things in life. The original owner of KFC started KFC at a late age, so I believe he had youth.
As one matures emotionally and spiritually, I believe it is a natural process to lose some of the youth. Personally, I can't help but see it as both good and bad. And, this loss of youth (i.e. the loss of childlike wonder) is much worse than any loss in physical youth. At the same time, one "grows" through emotional and spiritual maturity, and through this we also develop our into full potential, gain perspective, and gain a deeper understanding of the big/more important issues of life (world issues, spiritual matters, etc.).
I reviewed some verses on "youth" in the Bible, so I did not interpret them in such a way where "youth" is the more desirable stage in life.