I have lived the past 13 years in 3 tiny towns (under 100); just go to the bar every night. Made both good shallow and good deep friendships like that while I was over 30 most of that time. Mind you that was in another country with another language than my native. Personally I get depressed by big cities much faster; people I met when living in those are far (acting) busier and so need to keep things shallow.
Also, I seem to have some thing that people find weird; I can make and sustain deep friendships via internet chat alone. I have friends I speak to every day, who would do anything for me and I for them who I do not even know what their voice sounds like. I have started, ran and sold companies with friends who I have never seen or voice chatted to.
The digital equivalent of those places are Facebook groups. meetup.com, although their mobile redesign is a complete joke. Linked in groups. In the UK we have gumtree, probably Craigslist in the US. There's probably more, just sounds like you never looked.
You're looking for digital bulletin boards for your area, groups need new blood as much as you want to find them, you've just got to look.
Plus, of course, the local college night classes.
I just searched meetup for my costal corner of a home town of 20k in the UK and it had a bunch of meetups nearby, so it's pretty easy to find stuff.
Except: Church organizations. And it seems such organizations prefer that you are also Christian or are willing to fake it.
Furthermore, the poorer one is, the less likely one can take advantage of these things, especially in a town of around 50k or less.