I'm 21. I've had both old (over 65) and young (under 30).
I personally enjoy the under thirty year old doctor SO MUCH BETTER. I feel like she's in the trenches with me, so to speak, whereas the older woman I saw seemed to be looking down upon my life from a higher -- better - place.
I suffer from anxiety, socially and personally, and depression. SO I cant tell you what's best for you unless you suffer from the same thing, and even then if you're older than me, or at a different place than I am (as most 21 year olds I find are), then I can't really say.
But in my experience, I want someone to talk to. And I'd rather talk to a friend than a doctor, and I can honestly say the 30 year old doctor would be my friend, irl.