Yup. Though I did have a rather (nothing as bad as others, but bad enough) stressful childhood, it presented at 20 for me (at least that was when the first massive depression hit... there were a few things earlier that made me wonder. Still, let's count 20 as the official start.)
Soooo many years undiagnosed so it *did* feel like the norm.. miserable, but the norm. A bit like a bad childhood -- when it is all you know, it may well seem normal(!)

One of the tricky things with BP is that there can be periods of remission. That certainly doesn't mean it won't come back, though we like to convince ourselves of that at the time, yeah?

I don't think it's really possible that someone could "stop" having BP, so to me, that is a moot question. Also, untreated, it tends to progress and become harder to treat. That's not worth it. Like blueoctober, I've discovered that when I've messed with my meds ("but everything's going so great now!"), I quickly realized just how bad I was w/o them.

Good question though, widgets. The starting at later ages thing has been very much on my mind lately. Sure, it's much less common, but intriguing to be sure.