What are the implications of early versus late sexual development for girls? Is one more beneficial than the other?

The early onset of puberty presents children and their parents with predictable challenges. Since girls typically develop earlier than boys, those who mature first among female contemporaries are miles ahead of everyone else. This can produce serious problems, because it isn't socially advantageous for nine- or ten-year-old girls to be boy crazy, have monthly periods, and develop breasts when their friends are still thinking and acting like children. A precocious girl may also lack the emotional maturity to handle the attention she garners from boys. She is at greater risk of being sexually active, contracting a sexually transmitted disease, and experiencing an early pregnancy.

There are additional dangers for early maturing girls: they are more likely to be aggressive, socially withdrawn, and moody, and they experience depression at a higher rate. They also have more problems in school and are more likely to smoke, use alcohol, and do drugs. They are also more prone to breast cancer as adults.⁽¹⁾ So in answer to your question, girls typically do better when they experience menarche at about the same time as their girlfriends, or perhaps even shortly thereafter.

This much is certain: girls who experience menarche when they are younger than their peers need close supervision, careful guidance, constant reassurance, good medical care, good nutrition, and an abundance of love. 

Footnotes: 

1. Diana Zuckerman, “When Little Girls Become Women: Early Onset of Puberty in Girls,” National Research Center for Women and Families; article first appeared in The Ribbon, a newsletter of the Cornell University Program on Breast Cancer and Environmental Risk Factors (BCERF) 6, no. 1 (Winter 2001).