Bureau of Justice Statistics,,,
Homicide trends in the United States,,,
www.ojp.usdoj.gov/bjs/homicide,,,
Date of release: January 17 2007,,,

Percent of Homicides with Multiple Offenders by Offender Age,,,

 ,Offender Age,,
Year,14-17,18-24,25+
1976,26.4,19.2,6.2
1977,25,17.9,6.5
1978,24.5,18.4,6.7
1979,26.6,19.5,7
1980,23.9,18.2,7.2
1981,27.1,19.2,6.9
1982,30.6,18.9,7.4
1983,25.3,19.3,7.5
1984,26.3,18.8,7.7
1985,27.3,18.9,7.6
1986,26.5,19.6,7.7
1987,24.5,19.6,7.8
1988,29.1,23.6,9.3
1989,28.5,22.2,8.9
1990,32.2,25.2,9.8
1991,31.5,26.3,10.5
1992,29.6,25.4,9.5
1993,30.9,24.9,9.1
1994,31.8,24.5,9.5
1995,31.9,24.8,9.8
1996,33.8,26.9,9.6
1997,31.9,24.9,10
1998,31.1,26.4,10.6
1999,29.6,24.3,10.9
2000,31.6,27.7,11.7
2001,34.8,27.2,11.4
2002,33.8,26.6,11.8
2003,33.8,25.1,10.7
2004,34.7,25.9,12
2005,35,27.7,13.7

"Source: FBI, Supplementary Homicide Reports, 1976-2005",,,
See the methodology section in 'Additional Information About the Data' for weighting and imputation procedures used.,,,