This small group is the "chronic 6%," a term used to refer to Chronic Juvenile/Delinquent Offenders.
What characterizes these juveniles is that they do not typically "grow out" of criminal behavior. Rather, they continue as adults. Out of all delinquent conduct, a large number of offences is attributed to these youths.
The term "chronic" in this context means that they repeatedly break the law over a long period of time.