Anecdota:

Mixed-age groups of home-schooled kids (e.g., at co-op events) tend to just play, with the older ones typically watching out for the younger kids, whether they are siblings or not.

Mixed-age groups of formally-schooled kids begin by setting up a hierarchy that usually depends on grade in school* and then is continually refined.

* H-S kids in a mixed group are immediately marked as outsiders by this question. They either don't know or operate at different levels in different subjects.

Home schooled kids are typically in groups of far, far smaller sizes. The social dynamics of a high school with 2000 students is naturally going to be different than the dynamics of a home school of a dozen or so.

I think the point is that the ingrained dynamics continue even when the context / group size evens out later.

>Has that been studied?

Depends on how studious you are.