When puberty is well ahead of schedule -- termed early puberty, premature puberty, or precocious puberty -- it’s usually not medically worrisome, but it needs to be checked out. Body fat can sometimes affect when puberty begins, so if your child is overweight, puberty may be earlier. Premature puberty is more common in girls, who can start developing breasts and pubic hair before seven or eight. In boys, premature puberty can cause a deeper voice, growth spurts, and acne before nine. If your child may be experiencing early puberty, talk to your doctor about evaluation and testing. While it certainly doesn’t mean there’s a medical problem, it’s a good idea to be sure.
From The Smart Parent's Guide: Getting Your Kids Through Checkups, Illnesses, and Accidents by Jennifer Trachtenberg.

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The Smart Parent's Guide: Getting Your Kids Through Checkups, Illnesses, and Accidents
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.