Which is of course supposed to be civil if nothing else.
Anything less is realistically a lack of human nature, often still remaining along a not-completely-civilized continuum.
So _human nature_ should never be used as an excuse for uncivil behavior, and further progress can best be made by recognizing _lack of human nature_ as the problem instead.
This should be obvious to behaviorists but once you go there you've got to figure lots of them would need more _common sense_, except there still remains some prevalence to _common lack of sense_ . . .
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.
Depends on how studious you are.