America's Military Population is a report published by the Population Reference Bureau in 2004. This report discusses geographic distribution, recruiting, leaving the military, personal characteristics of service members, and military families.
The Deputy Under Secretary for Military Personnel Policy website provides Population Representation in the Military Services reports for 1997-2007. The reports discuss age, race, gender, income, education, and marital status of military personnel.
Selective Service Induction Statistics are available on the Selective Service System's History and Records website. Statistics for specific years are available for 1917-1973.
Civilian Personnel statistics are available through Fedstats.