What's wrong with having fun on your birthday? That's like saying you can't celebrate Christmas with gifts from Santa, getting into the Christmas spirit (being nice, etc.), or whatever, just because the reality of the situation is that Santa isn't real and most people are [insert expletive here] (even during the Christmas season--I can't count the amount of times people have almost smashed into my car trying to steal my parking spot...it's really, really sad that people behave that way, especially at Christmas. Gotta get Timmy his Furby or Pokemon or whatever it popular nowadays, I suppose).
Anyway, I'm in my 20's, and my brother is in his 30's, and we still like to have fun on our birthdays just like when we were kids. It's a tradition that never really disappeared with us as we aged; we didn't let real life steal away the fun we had as kids. As such, we still get fun themed cakes, go out to a restaurant for dinner (although, the restaurants have gotten a lot more expensive now--no more McDonalds birthdays, but rather The Keg or Red Lobster, or something along those lines), get each other all sorts of presents, and just all around have a lot of fun. Now, it also includes a magic jamming session, since we both enjoy magic.
For my brother's birthday last year, we got him a regular cake, but decorated it with cheap little magic props (like, those cheap magic kits you can buy--that type of stuff). He loved it! And my cake from the year before was a Shrek cake (no, I'm not a huge fan of the series--but the cake had a castle (3-D castle prop, at that) on it...
a castle!!!). It's always fun times.
It's nice to have a break from the norm. I spend most days just hanging out with friends, going to bars, etc...so it's nice to do something different for birthdays. Hence, we keep with tradition and have an old school fun birthday, which includes a nice dinner and a fun themed cake, as well as some quality family time (since we're all adults, and thus live our own separate lives; family time becomes a lot more precious when you don't live together, anymore).