Hard to tell. The origins are really foggy so I couldn't tell you.
Leopard (though I didn't know her exact form/personality at the time) has been with me as long as I can remember (I'm 21).
Mars too has been with me for a long long time, my first distinct memory of him I was no more than 7 or so. Could he have been with me from earlier? Probably. Before 7 (which is when most of whatever caused us to be multiple happened---don't have access to those feelings/memories---at least not coherently enough) everything is fuzzy. He 'disappeared' from me for many years (and I suspect is to blame for my many years of what I thought were gender issues) and I think for a while I lumped him in with Firebird (she was my catch-all for everything that 'wasn't me' for a while). He resurfaced (after I was in therapy for a bit) all like 'errr....dude remember me? I'm not part of that crazy &^$#'
Firebird has been like I said, more of my dumping ground for all the miscellaneous 'alters' that I had---namely anything that 'wasn't me'. Which pissed her off. She at one point like I mentioned was a catch-all for the little one, leopard and Mars. Which caused her a lot of stress because she simply wasn't all these things.
Little One (hasn't named themselves yet): again, has probably always been there. I don't know the specifics.
My alters were always sort of there in my head. The hard part for them was making me see the distinctions between them. I used to think they were all 'just not me'. Yet whenever we'd ask ourselves what do I look like/feel today whoever was 'in the driver's seat' would show their self-image---which wasn't my own.