I've observed children who have had tremendous close friends in childhood but were unable to recreate that in adulthood. Sometimes it's easier to make friends when you're 5.
Children have school. School gives you a shared experience to talk about and time to talk to others, both through actual coursework and play. Children are handed the tools to possibly make friends and they aren't even old enough to have decades of baggage and anxiety yet.
As an adult, you have to create those conditions. For many, work serves this role. Hobbies and regular activities (bowling, for example) help. Depending on the person, it can be online (Met my spouse this way - a silly online game back in the later text-based, formulaic MMORPG era). And you are a lot busier as an adult with more responsibilities filling your time. Of course it is harder as an adult.
I got a lot of flak for going to a high school play in my late twenties. I had played D&D with the kid and his mom every week for years. He was great to hang out with when he was 14.
But it seems hard for many adults to play with children, so it becomes this anomalous thing, even though I’m fairly certain it’s just something we’ve convinced ourselves adults “don’t do”.
Tag is still fun, whether you are 7 or 37.
Q: How do you make friends in Boston? A: Same way everyone else does. In kindergarten.
"We've got two seasons: summer and construction" - do you have that one too?