Typically, the age at which the development of sexual characteristics occurs is 10-12 years old for girls, with an average age of 11 years, and 12-13 years old for boys, with an average age of 11.5-12 years. Girls show accelerated breast development and boys show enlargement of the penis and testicles. If your child has breast development before the age of 8 and menstruation before the age of 10 for girls, or enlarged penis and testicles before the age of 9-9.5 for boys, be alert to “precocious puberty”!