My daughters have known about getting a period from quite a young age. My 11 yo got her first period last year but it only happened the once, after that I have noticed discharge on her underwear but I know her body is obviously getting ready for when she'll start getting her period regularly. I've chatted openly about things with my 11 yo too and I let her ask me questions. My 11 yo also started sex ed this year at school so I feel like it's all fallen into place quite naturally and I've been able to elaborate or explain things that she's asked about. But she has a very healthy attitude to it all because I haven't turned it into a taboo subject and I have always been very open and honest with her, so to her it's all a very natural process.