I don't know if it's because of an evolutionary standpoint, or perhaps just because when we're young and move from class to class and school to school, we just get in contact with like-minded people of similar age on a regular bases, whereas once we're adults, we (most of us) no longer find ourselves in a natural environment where this is the case. Lack of exposure to like-minded people might be why our social constructs became more stable and stale when older.
I think the latter seems more likely to be the biggest factor. But I do think that actively searching for when young and keeping safe what you have when older rings true as well. Not really for me. I don't think I ever really actively searched for a place to belong amongst others. I've had plenty of friends, but in that sense I've always been a bit of a loner. If my friends liked some movement or clique (say music/dresscode/etc), I wouldn't automatically follow just to belong. Unless I also genuinely liked said movement. Through the years in school, my friend base got overhauled a couple of times as a result as well.