I normally have this weird feeling that it should feel different but it doesn't.
After my next birthday (20) I get this feeling that I get a lot less "significant" ones (like 10 you reach double figures, 13 you've become a teenager, 15 you can watch 15 rated movies, 16 you can learn to drive and have sex, 18 you can vote and drink, 20 you stop becoming a teenager). From then on I feel it would be like "woo, I'm 23... well, what now?".
Also, a lot of the age restrictions didn't really mean much to me when I reached them. Like when I turned 18, I'd been drinking and watching 18 movies for a few years already, now I was just allowed to do them in pubs/cinemas (heck, I'd even been drinking in pubs for a couple of years by that stage).