My 30s were so so so much better than my 20s, it wasn’t even funny. I partied practically non-stop from the point my 1st marriage ended in my early 30s, to when my 2nd one began at 41. I made tons of new friends, dated around a lot and had the best time of my life.
Now that I have 2 kids, a stressful job and my money evaporates the moment it hits my bank account, I also have lost all my friends. The only ones I have now are other adults with kids around the same age as my kids, because that’s pretty much the only time you get to socialize with other adults.
The secret is: hang out with people younger than yourself and/or get involved in a scene. I’m a musician so I just went to shows or met people at mine. It doesn’t have to be that though, I also joined a volleyball team and while I stunk up the joint, I also got to meet a bunch of cool people. Really most friendships are based on proximity and common interest, so if you’re into movies, go to movie festivals or special showings. Go to meetups, get involved in a political campaign (if you’re political).