As adults we're constantly interacting with people younger and older than us by multiple decades. Even as college students we're in classes with those younger o

As adults we're constantly interacting with people younger and older than us by multiple decades. Even as college students we're in classes with those younger or older by multiple years. The modern age-cohort school system is the historic exception. Prior to it single-room schools were common. And prior to those apprenticeships in which some of the apprentices or journeymen were older and younger were common. Heck, in my neighborhood kids were older and younger. I remember getting advice from an older neighborhood kid when beginning elementary school (the part I remember is that lunch boxes weren't cool, and to use a paper bag instead). You're right in that being the single odd-ball is unusual. But I think it would be better to just make partial and full grade skips more common than to make them rarer.