Most violent behavior begins in the second decade of life. Adolescence is a time of great change and vulnerability, which can include an increase in the frequency and means of expression of violence and other risky behaviors. Serious violence begins mostly between the ages of 12 and 20. The peak age of onset is 16. Violent youths commit a remarkably high number of crimes. These young people (both male and female) average 15.6 rapes, robberies, aggravated assaults, or some combination of these crimes over a 16-year period.