its a bit hard to explain BUT
The increased levels of sex hormones result in physical changes some of them , including maturation of the breasts, and other places that im not gonna say. Normally, these changes occur sequentially during puberty, resulting in sexual maturity whic hmost ppl think only boys have but most girls do to . ...
Genetics can cause , Puberty to happen earlier in girls whose mothers matured early. so it could be earlier then teenage years depending on when the mother started