Actually there's psychological explanation why people want to belong to a group. Then of course, these groups label other groups for some reasons: for example what kind of clothes they wear, or what kind of music they do listen to.. and so forth and so on.
Most people, no matter how old or young, want to belong to a group - usually to a group of similar kind of people. This will make these people feel themselves welcome and gives them feeling of safety. Of course it is easier to hang around with people who have same interests. Suggested reading: Maslow's hierarchy of needs.