Your school friends talk less and less the older you get and then start their own families and lives.

Your marriage/relationship friends are fine, but it can't be your only identity

Your work friends can go either way but you don't want to solely rely on that.

The best thing I did in my 30's was try out new hobbies until I found one I love. Join a rec sports league, or some kind of club. Take a class etc...

I started doing improv comedy 5 years ago and basically all of my non work/relationship friends come from there at this point. The thing with doing something like that is everyone is signing up for those things for the same reason for the most part. It's a lot of adults looking for a new thing to do outside of their norm and meeting new people.

It's awkward as shit to meet people as you get older, a thing like that throws you in a situation where you all have to meet each other and talk to each other.