Yep, I've turned 25 ... no more early-twenties for me, right now I'm smack-bang in my mid-twenties, and next year I'll have started my late-twenties and the descent/ascent (depending on your outlook) towards 30.
I guess now's the time you start looking forward to your birthdays less and less, or that birthdays become less important ... that's quite true I think ... I still like getting the presents and cards, but now - particularly turning a quarter of a century old - it starts becoming a case of (like many adults) "ah geez, another year older, where did the time go?" and so on.
That said, I'm still in my twenties, so it's not all bad by any stretch of the imagination - but my point is that this is probably where it all starts to get increasingly depressing and grown up as the years tick by (gradually, of course).
But then again, the 20s are the new teens, and the 30s are the new 20s, and the 40s are the new 30s ... so there's always an excuse to enjoy fart jokes, and in the end, that's all that really matters isn't it? ... ... well ... ... hmmm.