sorry if this is a bit too gtky'y for your tastes...
My 21st birthday was initially rather tame and staid. I went to my summer work, did my computer thing, and came back home. I boiled some frozen dumplings, and ate them with one of my good friends. After that, he and I then went out to the beer store, and he got his usual while I pored over the list, trying to figure out what would best fit both my budget and palate. I eventually settled on Yuengling's Black and Tan. Nothing quite matches the giddy feeling of finally being able to stand at the counter and order a case of whatever it is you'd like to have.
Following that, I sat around for a bit, waiting for my beer to get drinkably cold. I caught a couple of episodes of Law & Order, and then decided that it was time for that beer. This fueled, of course, a betterment of my drunken state. My friends and I took to a local bar and they proceeded to fill me with all manner of mixed drinks, followed by a shot or two. I was apparently removed from the bar after managing to puke into an empty pitcher.
Now I look at the rest of my life and I see no major birthday-related milestones to look forward to. I would almost say that I'm disappointed that this has happened, but really, what has happened is that I turned another day older. And that's all that has happened, every day, since I was born.