In this study the delinquent histories of 270 gang and 104 group offenders were selected from the Philadelphia Police Department's juvenile records for the periods before and after 14 years since most gang involvement begins at that age. Analysis of offense data showed that while the proportion of violent offenders age 13 and below was approximately equal for all subjects, at age 14 gang offenders began to accumulate more violent offenses than their counterparts. Finally, 27.8 percent of the gang but not one of the group offenses at age 18 involved violence. Gang members 15 years and older were much more likely to commit violent rather than property crimes. A similar pattern emerged regarding police contacts in which participation in gangs increased the likelihood that juveniles would violate the law. The review of the criminal histories also revealed that gang members past age 14 were apprehended more often by the police than group offenders and for more serious crimes. The theory that the earlier juveniles begin criminal careers, the more likely they are to commit serious offenses at a later age was tested by studying the ages at which the sample subjects first violated a law. However, no appreciable differences between gang and group offenders were discovered. The initial age of subculturalization into a gang occurs at 14, peaks at 16 or 17, and declines sharply by 18 years. Critical points of subculturalization are slightly different for group offenders, who appear to be vulnerable to group influences at an earlier age than gang members. These observations should be considered when formulating a prevention program, and the best moment for intervention might be the age immediately prior to the initial susceptibility to gang or group pressures. Statistical tables and footnotes are included. For a related article on gangs and groups, see NCJ 72871.