I think it's more a matter of people tending to have fairly "loosely bound" friends through their 20s. People they went to school with, people they didn't mind

I think it's more a matter of people tending to have fairly "loosely bound" friends through their 20s. People they went to school with, people they didn't mind hanging out with when there was nothing better to do. But not necessarily the close friendships that are indispensable. As people get busier and as the novelty of "just hanging out" wears off, people find they weren't as close to a lot of their "friends" as they thought, and those friendships sort of fade into the background. If you don't have a way of making new friends, that's going to set the trend for your friend group overall as you age. People who are younger tend to be thrust into a lot more circumstances where they are in touch with a lot of peers regularly than adults, especially school. Adults have work, but there tends to be a much smaller group of peers there in comparison, and not everyone is good at making friends through work. For my own personal experience though, at 40, I don't think it's particularly hard to make new friends, you just have to actually do things and meet people every once in a while. I'm pretty much a homebody in a lot of ways and yet I continually meet new people and make new friends year after year. A lot of that is meeting friends of friends, and of engaging in things like volunteering and participating in various hobbies and meeting people that way. Strangers aren't going to come walking into your living room and demand that you be friends, you have to meet people, engage with them, and actually be willing to form new friendships.