I've reached a age where I have to answer "define young". Early thirties is weird. All the people I respect and learn from are still older than me, but not old old.
I'm in the same boat and I definitely still feel young. I mean, I'm fundamentally unchanged from when I was 16, and still like the same stuff like video games, computers, and so on. I have also always looked kinda young, which probably allowed me to propagate my youth until very recently. The biggest change I've noticed is not anything internal to me but how other people act around me; they talk to me like I'm an adult (which can be either good or bad depending on the situation), and the cashiers at the stores have stopped asking for an ID when I buy liquor (before 30 they still did!). Also, people don't give unsolicited advice as much as they used to; I suppose they think I know what I'm doing or don't care. This is definitely bad, because my modus operandi for life has always been to try weird stuff my own way and count on other people to stop me if I'm doing something completely stupid.
The "define young"-range seems to be a sliding window centered around your current age.
Perhaps at some point there will be very few older than you and perhaps then the sliding window will be offset accordingly, but at least for me (almost, almost 40) I'm still well within the "define young" range from my own point of view.
It didn't occur to me that I was old until your comment. Thanks!