I must say, it's pretty underwhelming. Once you reach 21, birthdays are actually kind of boring, nothing all that special happens. When I turn 25, I'll be able to pay less for my car insurance, though I'm an infrequent driver so my rate is pretty low anyways. Beyond this, there's nothing really all that exciting, primarily depressing birthdays until I get old enough for social security.
Now that I'm 24, it really brings into perspective a couple of things. I'm pretty much at the peak of my athletic prowess, and more or less it's all downhill from here. BRILLIANT.
Birthday plans: go into lab and do lab work, I have a birthday dinner planned with a small group of friends, and after that I'll probably hang out with a couple and watch a movie. My girlfriend is currently in Minnesota and will be there until next weekend, so it'll be a somewhat lonely night spent doing work (ugh) in my apartment. It's sad, but I'm getting too old to go out and hit the bars. Even 1 hardcore night at the bars takes a full day of sleep and rest to fully recover.
Either way, should be a relaxing birthday. The next in a line of non-interesting and pretty much uneventful birthdays until 65 ><