I remember my best friend had pubic hair when she was 9-10 (much younger then me) and didn't get her period until she was 16. I only got pubic hair when I was 11.5 and had AF before I was 12. I think a combination of signs such as hair growth, breast development, body odour etc together would be a better indication of when a period would start then just one thing. It is surprising but isn't it? My DD is 10 and she seems way to young even in 2yrs time its hard to think she could have her period.