I don't feel like answering all your questions, but I'll give you a couple of things I've noticed. They might be specific for me, but I don't think so.
1. You know that feeling you get every now and then, where you're absolutely buzzing for no particular reason but especially when you're eg out on the town with friends? That feeling where you want to run around and yell and scream and do stupid things? An irresistible physical high, almost? Well that disappears.
2. Okay I was going to give you a few more but I think they'd mostly be bitterness. You'll probably learn a lot about bitterness as you get older. It's a different beast from the younger equivalent, feeding as it does on far more regrets.
3. Alright I'll give you one more. Making close friends? Way harder. Way, way harder. So make sure you get yourself a good set before you're 25 because chances are they'll have to last a long time. I'm not quite sure why this is. I have theories but they're pretty fuzzy.