Train jiu jitsu. I’m currently in Paris for the week with 12 of my teammates who are also now some of my best friends, all of who I met after I was 30.
I'm glad someone mentioned JJ. It's easily one of the best ways to make friends as an older person. It's also a great way to be exposed to people outside of your usual social bubble. I'm not sure I know of another activity that cross cuts society so effectively.