Doctors use the term “precocious puberty” for children who enter puberty significantly before the normal time frame. The normal time frame for girls to begin puberty with breast development is around age 10. If a girl begins breast development before age 8, it is considered precocious puberty. The normal time frame for boys to begin puberty with testes growth is about 11 1/2. If a boy begins early than age 9, it is considered precocious puberty.
If you are interested in hearing a doctor discuss precocious puberty, you can watch this 10 minute discussion from the Khan Academy: