I'm posting in AIBU because there's so much traffic here and I'd love to get some advice on something.
I have 3 DC and the youngest is DD (9 almost 10). In some respects she is 9 going on 7, and in others she's 9 going on 15! My other two are boys so this is the first time the issue has come up.
I had my first period when I was 11 and I believe my DM started pretty young too. My own DD is showing absolutely no signs of puberty and still hasn't reached that stage were there's any significant difference between the boys and girls in her class. They all still play together and the boy/girl divide that sometimes starts to happen at around that age doesn't seem to have started yet.
Every time I broach anything even remotely linked to her and puberty, she changes the topic. I think because she has older siblings, she knows it's a "thing" but she has zero interest in it and is not in the least bit shy about her own body. Happily parades around the house looking for her pjs and towels when she's getting ready for the shower or getting dressed. Respects other people's privacy but doesn't feel the need for it herself just now.
Part of me wants to not force the issue and instinct tells me she's not quite ready for any "preparation" but another part of me worries that I'm thinking that because it's ME who's holding on to her innocence. I'd hate for her to get her period out of the blue and feel shocked and unprepared.
If it's relevant my own periods were HORRENDOUS with endometriosis that went undiagnosed well into adulthood. My DM was the same and I worry that this is what lies ahead for DD.
If you were in my position when would you start talking about periods? I can't mention my own because I have a coil and haven't had a period for years. Otherwise that would be a brilliant way in.
So....
YABU - worse would be that she's unprepared. Start talking about it now.
YANBU - give it a while and try again in 6 months time.