I don't want to be old.
Oh, grow up man. Oh, wait a second... that's your problem, isn't it?
Joking aside, I don't see what the problem is. Why, I'm already a full 23 as of a few weeks ago. Big whoop. 23. I mean, I feel a heck of a lot older usually (I guess there's a reason people have told me I have to have more fun and let loose, but meh) - why, I even have the grey hair to prove it! Okay, so it's a streak, and it's genetic, but it's there nonetheless.
So, 20 still seems rather young. Stop fretting: 23 will come soon enough.