"Of the 2.3 million inmates in custody, 2.1 million were men and 208,300 were women (table 9). Black males represented the largest percentage (35.4%) of inmates held in custody, followed by white males (32.9%) and Hispanic males (17.9%).

"Over a third (33.8%) of the total male custody population was ages 20 to 29 (appendix table 10). The largest percentage of black (35.5%) and Hispanic (39.9%) males held in custody were ages 20 to 29. White males ages 35 to 44 accounted for the largest percentage (30.1%) of the white male custody population.

"The largest percentage (35.9%) of the female custody population was ages 30 to 39. Over a third of white females (35.9%) were ages 30 and 39. The largest percentage (36.8%) of Hispanic females in custody was ages 20 to 29."

Source

Sabol, William J., PhD, and Couture, Heather, "Prison Inmates at Midyear 2007," Washington, DC: US Dept. of Justice, Bureau of Justice Statistics, June 2008, NCJ221944.