It starts well before 30, both in the article, and in the developmental psychology subject I was taught over 25 years ago. Back then I was taught that people fo

It starts well before 30, both in the article, and in the developmental psychology subject I was taught over 25 years ago. Back then I was taught that people form cliques of roughly 8-15 people in their late teens to early 20s, and then those cliques dissolve. This stuff has been known for a very long time, and it's not "cities are bad", it's "people pairing off and moving out of their parent's place". Then for a bit of variety, throw in the others that disappear because of personal politics, move to another continent, or even simply die. When you're at school, you all live near each other. As you pair off and move out, you don't live near each other anymore, and you're also not spending large amounts of the day together. Similarly, your clique is learning how to be adult together. How sex works, how relationships work, how earning wages and paying your way works. It's an intense time. By the time most people reach their mid- to late-20s, they've got most of that sorted out. When you're 20, relationships breaking up amongst your friends is new and intense and interesting and gossip-worthy; when you're 30 it's boring and people are tired of hearing the story. Not so much opportunity for the heart-to-hearts that help make strong friendships.