Approximately one in eight women will develop breast cancer at some point in her life. Girls or boys who develop much earlier this age, may be exhibiting signs of precocious puberty, which is a medical condition that needs to be managed by a pediatric endocrinologist. Asked in Menstruation When should a girl start her period for the first time? Asked in Beauty Breast girls what age develop age does girls get breasts? Until then, the generally accepted age was 11 or so.