When did I know? When I was dx'd. Sorry, bit of a smart**. But also true, as you will see.
Dx'd mid 40's. First out-of-nowhere massive depression at 20. Like Christina says, then, it just wasn't like it is now. (This is where I usually go into my little rant about the times and levels of denial we're talking. Will refrain. You're welcome. )

But when did I know….I always knew I was… "different". Symptoms…. yeah, there was bunch of childhood stuff I can look back and wonder, but I don't "count" any of it, because it's too far removed to be able to make an accurate assessment of. So I "count" it starting in earnest at 20, because it was an undeniable watershed event. Between that and actual dx, I knew there was "something" very wrong with me, but was terrified to know what (because it bore too much resemblance to my mother's behavior, and there was nothing I DID.NOT.WANT more than that. Whatever "IT" was! Coming from a long line of Sweep It Under the Rug-ers, there was no way I'd consider a psychiatrist, especially if they were going to say what I did not want to hear and, almost as bad, ask me to talk about stuff!

After many many years of having the patience of a saint, my ex insisted I get help. Had a major misadventure after misdiagnosis (MDD by GP), then the right one, BP, by psychiatrist. I hadn't known diddly about BP till right in that time frame.

So, really, I couldn't have known much earlier than that, since I didn't know about BP! Despite being a walking talking textbook example, lol.