What I find ironic is that this interdeterminacy also appears to apply to me

Same here. I think it may be a geek/fannish thing: as kids/teens our research into our hyperfixations and/or fear of screwing up socially can get us read as unusually mature; as adults the same thing can make us seem younger than we’re supposed to be. In my late teens to early twenties either could happen, depending on the situation and what I was wearing.