Although it can start at many different times, the most common age group of onset is actually late teens/early 20s. So, no worries, you're right in there!

Oh, I can sure relate to that! It ran from being loud and running the show ("quiet down!!!") to not talking at all (In kindergarten, I'd tap on the teacher to hand something in, eyes averted and take off. I don't recall having friends or socializing at all early years at school.) In second grade, I have a very distinct memory of realizing I wasn't like the others. It was a pretty dispassionate observation of a simple fact. I didn't get what made them tick or how they interacted. Whatever. I lived in my own head the majority of the time.

I don't make anything of those years (except as proof I've been a weirdo for a long time, lol) as anything more than interesting observation/recollection. There is no doubt however about when things got serious for real. Beginning of my third year of university, a massive depression hit out of nowhere. And I mean nowhere. I was utterly baffled what was happening, but mostly WHY. There WAS no why! WTF?! People talk about school pressure blah blah blah. Pffft. Nope. I was having a GREAT time! It was preceded by literally the best time of my life, hands down. Friends, social life, bands, dancing, woot! Sure, with what I know now, it probably was hypomania, the high before the crash, but I can't pinpoint it, so I don't. (The time span would've been the last couple years of h.s. up till the tsunami of depression at 20. That's a long time, so… I dunno. It was a real departure from my life till then. I do know I was calling the shots -- and despite having one main group, got along with just about everybody, could hang with any group and well, yeah. Many years later, my h.s. best friend's mother thanked me for being her friend. Ohhhh, you wouldn't be saying that if you only knew! )

Anyhow…. it probably was some time in there, but I only officially count it as BP with the huge depression (stopped going to classes, barely left room for months, had to drop out) at 20. (Oh, how common of the common… ) Why? Because it had a definitive start, came out of absolutely nowhere and there was nothing else it could be attributed to or confused with.

Then followed decades of being all over the map, literally and figuratively.