People are cynical about your idea, but it's not like it has never been done in society. There are in fact apartment complexes that focus on elderly demographics. Retirement homes are just that. What's a school but a bastion for a bunch of kids of similar age to socialize together (and should a random adult wander onto it, there'd be a lockdown).

Urban planning around these situations would be nice. But the very least we could do is build more third places that tries to attract that last middle demographic to come together; that mid 20's to 40's adults too old for college campuses but too young (and not on social security funds) for older gatherings. That's sort of what some bars can be, but a wide open space with less pressure to spend $15 on a "cheap" beer would be nice.