Precocious puberty is when a child's body starts to develop and change into that of an adult's body before they are 8 years old for girls and 9 years old for boys. This means that their body starts growing and changing earlier than it is supposed to. It can cause a child to grow taller more quickly, to start developing breasts, to get hair in places they shouldn't yet, or even to start having periods. It's not something to worry about, though, because there are doctors and treatments that can help.