By federal law (10 U.S.C., 505), the minimum age for enlistment in the United States Military is 17 (with parental consent) and the maximum age is 35 (Note: Congress changed this to age 42 in 2006). However, DOD policy allows the individual services to specify the maximum age of enlistment based upon their own unique requirements. The individual services have set the following maximum ages for non-prior service enlistment:
Active Army - 42
Army Reserves (Including National Guard) - 42
Active Air Force - 27
Air Force Reserve (inlcuding National Guard) - 34
Active Navy - 34
Naval Reserves - 39
Active Marines - 28
Marine Corps Reserve - 29
Active Duty Coast Guard - Age 27. Note: up to age 32 for those selected to attend A-school directly upon enlistment (this is mostly for prior service).
Coast Guard Reserves - Age 39.
However, prior service enlistees can receive an "age waiver." In most cases, the amount of age that can be waived depends upon the amount of time the individual previously spent in the military. For example, let's say that an individual has four years of credible military service in the Marine Corps and wants to join the Air Force. The Air Force could waive the individual's maximum enlistment age to age 31 (Maximum age of 27 for the Air Force, plus four years credible service in the Marines). For the Marine Corps, the maximum age of enlistment for prior service is 32, after computing the prior-service age adjustment.
For the Army National Guard, the maximum age for non-prior service enlistment is age 39. For the Air National Guard, it's 34. For prior service, the maximum age is 59, as long as the member is able to complete 20 years of creditable service for retirement by age 60.
Good article here: http://usmilitary.about.com/od/joiningthemilitary/a/enlstandards.htm