I met most of my current friends in my 30s. I think it's possible, you just have to prioritize it and put yourself out there in situations where you'll encounter the same people over and over again and can grow something over time because of it. Either sports, or games (board games has been pretty darn effective for me), or exercise (like going on hikes together), or volunteer work, or something.

Like you said, grad school has been useful because you're all going through the same situation and spending repeated time with each other.

For me, it was picking a handful of Meetup.com groups that interested me and going to meetups over and over and over again, until I became one of the regulars that knew pretty much everyone and they knew me. And then we started inviting each other to private events, and friendships grew from there.