I'm 20 and I feel okay. I'll be 21 this year, which isn't so much "bah I'm old" as "hey I could purchase alcohol anywhere!"
I don't think I'd
want to go back and be 16 again or anything. On the whole, 16-year-olds are kind of uh, annoying, sorry to any 16-year-old ZUers! I know I was an annoying 16-year-old. I feel like since then, even though I haven't accomplished a huge amount and on the whole I'm not very 'grown up', I'm still better than I was at 16.
When it comes to thinking about what I've accomplished I'd rather not.

I graduated high school with zero extras, dropped out of a TAFE course halfway (I earned the cert IV, at least!), then moved away from home to live with my boyfriend and housemates. Also I run a YouTube channel, and that is fun.
Sometimes I kinda get disappointed that I haven't "accomplished" much for 20, when other people my age are nearing the end of uni and going to start their actual careers and I just work in retail and play video games, but I'm working on it. Besides, I was completely dependent on my parents about two years ago, and now I've lived out of home for over a year, so that's a pretty big accomplishment for me at 20 (considering most of the other people my age live at home still).
I think I rambled. Sorry.
tl;dr I am 20 and that is okay. Sometimes it feels old if I'm hanging around younger people, or realising that my cousin who was born in 2000 will be in
high school next year, but man, my grandpa is turning 90 tomorrow and still kicking it (albeit from a chair, most of the time), so if that's anything to go by I think I have some time left.