I'm someone who didn't start feeling like an alien until I began to understand how different other people's experiences were, hence I checked "puberty". I also checked "other" because I feel like an alien at certain times, surrounded by certain people, but it doesn't match any of the options you give. Mind you, I've gone to a fair bit of effort to surround myself by people who think more like me, or who simply take that kind of variety for granted.

The hardest situations to cope with is when other people have not yet come to understand how different things can be for other people than them, and have a sort of collective delusion that they at least are all looking at stuff the same way. Because usually they aren't.