It really depends on what you mean by "young".
There's the physical/sexual aspect, which really depends on how well you keep in shape. I'm 42 now, but even at 25 there were people 50+ I found damn sexy. (plus they've got all those skills and experience, which is a big plus, but I digress). Mostly, I'd say 35.
There's the psychological aspect, ie when y'oure still supposed to make mistakes and think mainly of yourself, vs when you're supposed to know your stuff, and be thoughtful of others. I'd say the cutoff should be around 25; but it really depends on a person's experiences. My last relationship was with a 24yo (I was 35), and it failed basically because he never had to pull his own weight, and still needed much mothering. In spite of his many other qualities (he was intelligent, funny, affectionate, honest...), this "youth" eventually proved fatal to me. So cut off = 25, or when you've lived on your own, from your own money (earned from your own work) for a year.
Finally, there's the social aspect. It's a long journey to know who you really are, to stop being controlled by fads / advertising / peer pressure. It's a fine line vs being jaded or cynical, and some never get there, or overdo it. Knowing what's really important to YOU, as opposed to what you think is important to people you want/need to like you. Looking at my 18yo niece... she's right in that phase where she buys stuff 'coz that's what needed to be popular, clothes that don't really suit her but are the right brand... There's that debate about fashion vs style, she's on the wrong side of it right now. Same goes for activities, musical tastes... At some point in people get to realize that books, films, music, even TV series do not exist in vacuum, and that there's some very good stuff that's not known/advertized/popular, but still, better than 99.9% of the current productions. There's not cutoff age for that really. Hopefully the process in under way by 30 ?