Teens who have sex tend to think their friends are having sex, too -- even if they're not....Plus, teens tend to overestimate how many of their friends are sexually active. Bad combination.

So, if we can persuade teens that fewer of their friends are having sex, that belief may reduce their chances of having sex at a young age.

The younger a girl is at the age of her first sexual experience, the more likely she is to have a much older partner.

Girls who had sex later in their adolescence tend to have partners closer to their own age.

The younger a girl is when she first has sex, the greater her risk of suicide, alcohol and drug abuse, truancy, and pregnancy.

"Frequent parent-child discussions about sex and its dangers may prevent teenagers from engaging in risky sexual behavior."

"Adolescents who felt that their families were more supportive were less likely to have unprotected sex, and thus were at a lesser risk for pregnancy and disease."

Psychology Today: Teens and Sex