Because they're young.
D000293:Adolescent; D000328:Adult; D000374:Aggression; D002648:Child; D003220:Conflict, Psychological; D006801:Humans; D007398:Interpersonal Relations; D007604:Juvenile Delinquency; D010551:Personality; D011627:Puberty; D012723:Sex; D012931:Social Environment; D014481:United States; D014754:Violence; D014857:Warfare