I can legally drive, smoke, drink, vote, gamble, and join the military already. In fact I don't think turning 21 will give me any new privileges. For a literal answer, I'd probably wake up. Hopefully.
I don't remember the first thing I did was when I turned 21. It wasn't as much of a landmark birthday as it should have been, but maybe that was for the best.
If I "could" turn 21? Are you implying I won't be able to turn 21? Are you planning to off me the day before my 21st birthday? What kind of sick people are you!?