I've often thought this -- age stratification to our current degree is a relatively new (20th century?) phenomenon, and seems under-studied. I think the awareness came to my mind when I went to a number of mixed polka/swing dances back in like 1999 put on by the local Czech center. It was a rare mix of seniors doing traditional polka side by side with college students (who were brought in by the swing dancing) along with other adults, their kids, 5 year olds running around, etc... And then we'd do goofy dances like the Hokey Pokey or the Chicken Dance. (This was via the eclectic and fun band, "Brave Combo.") The entire mix felt weirdly... healthy.

One reason we chose a private girl's school for our daughter is that, along with traditional (horizontal) grades, it has "sister groups," which are vertical slices of girls from varying grade levels. These meet and socialize throughout the year.