Children tend the separate and differentiate during puberty. This is the main difference in socialization that cues all other socializations to start. Children that may have played together when they were younger will start to drift apart or unify according to gender, interests and attitudes.
Self-identification is one of the big parts that spurs this separation. When puberty happens, blossoming teenagers start to become more self-aware in terms of image, status and worth, and will begin to form solid identities for themselves. This takes a lot of trials and times, which is why teenagers go through "phases" to find what they are most comfortable with.
Once this identity searching starts to happen, teenagers tend to cluster together. They may become distinctly mean or start using different speech patters. This is also part of identity testing and serves to help bond them with their group and make it clear to the world that they identify as something specific.
Teenagers will also start going out more. This is partially due to media influence that pushes a certain level of activity and nightlife. Going out or staying out late at night helps teenagers learn boundaries and meet new friends while having fun and releasing pent up energy.