The incidence of co-offending, persons acting together to commit a crime, decreased with age primarily because individual offenders change and become less likely to offend with others rather than because of selective attrition of co-offenders or persistence of those who offend primarily solo. As male ages, they were more likely to offend alone, but most males continued to commit some offenses with others. Exclusive solo offending or exclusive co-offending behavior was uncommon at all ages, but there was a significant tendency for specialization in either solo or co-offending. The likelihood of recidivism was slightly less after offenses committed alone than with co-offenders. 49 footnotes, 11 tables, and 1 figure