OMG, I feel the same way, I'm turning 26 on Friday and the same thing.
I'm like when did I turn old and responsible? It's a great thing though because we are VERY fortunate to have our *ish together, some people are still partying, blowing through money, and acting like a teenager, I love that we have our lives together and still are able to have fun and party like the youngin's. =)