Teens want to be with people their own age - their peers. With peers, teens can be both connected and independent, as they break away from their parents' images of them and develop identities of their own.
Forming successful peer relations is an important part of growing up. Friends influence who we hang out with, how we dress, and what we do for fun. Friends also can affect our behaviors, attitudes, and beliefs.

The influence of peers - whether positive or negative - is of critical importance in teen's life. The need for acceptance, approval, and belonging is vital during the teen years. Teens who feel isolated or rejected by their peers are more likely to engage in risky behaviors in order to fit in with a group.

A powerful negative peer influence can motivate a teen to make choices and engage in behavior that his or her values might otherwise reject. Sometimes, teens will change the way they dress, their friends, give up their values or create new ones, depending on the people they hang around with.

Parents can support positive peer relationships by giving their teenagers their love, time, and encouragement to think for themselves.

No matter what kind of peer influence your teen faces, he or she must learn how to balance the value of going along with the crowd (connection) against the importance of making principle-based decisions (independence). And you must ensure that your teen knows that he or she is loved and valued as an individual at home.

http://www.focusas.com/PeerInfluence.html