The percentages, respectively, of men and women in "the active component of the U.S. Armed Forces," according to a brief overview in the November issue (PDF) of the Armed Forces Health Surveillance Center's Medical Surveillance Monthly Report. By contrast, the civilian population is almost evenly split, at 49.2 percent male and 50.8 percent female. According to "Surveillance snapshot: age and gender distribution of service members, active component, U.S. Armed Forces, compared to the U.S. population, 2011," the average age of male/female service members are 27/26. See the associated graphic for a detailed breakdown.
For an even finer-grained look at who comprises today's military, see the recently released report, 2012 Demographics: Profile of the Military Community (PDF). This report "present a synthesis of demographic information describing members and families in the military community. Active duty Service branches include DoD's Army, Navy, Marine Corps, and Air Force, and the Reserve Components include DoD's Army National Guard, Army Reserve, Naval Reserve, Marine Corps Reserve, Air National Guard, Air Force Reserve, and the Department of Homeland Security's (DHS) Coast Guard Reserve."