I would stay the age I am now. Only, I would want my actual age to say 30 as no one seems to take you seriously when you're 22, it's really annoying and it makes them look like a rude jerk. Old and wise aren't exclusive things.
But physically, I'd like to stay 22.
ETA: I remember hearing something like
When people are 22+, they wish they could still be 21.
... 30+, they wish they could still be 29.
... 40+, they wish they could still be 39.
But then after they get to 50+, they still wish they could stay 39.