Updated December 2017
It truly depends on whether or not you are a non-prior or prior service member, looking at enlisting into the active duty or reserve components, etc. In the United States military system, you may enlist into any branch at the age of 17 years old with parental consent, or 18 years old without parental consent.
Age waivers, although rare, are possible. Usually happening if one cannot be sent to basic training by their birthday, thus making them ineligible to enlist if they are then past the required ages, age waivers can be received for just a few months easily.
|Air Force Reserve||38|
|Marine Corps Reserve||29|
|Coast Guard Reserve||39|
|Army National Guard||35|
|Air National Guard||39|
In the United States Army, Army Reserves, and Army National Guard, the maximum age allowed to enlist and ship to basic training is 35 years old. The Army experimented for quite some time allowing anyone up to the age of 42 years old to enlist and complete basic training. On April 1st, 2011, they reverted this to the lower age limit.
In the United States Air Force, to enlist, you cannot be younger than 17 (18 for GED holders) or older than 39 years of age. To join as an Officer, you cannot be younger than 18 or older than 39 years of age. To join as a healthcare, law or ministry professional, you cannot be younger than 18 or older than 48 years of age. In the Air Force Reserves, the maximum age is 34 years old. In the Air National Guard, the maximum age is 39 years old.
In the United States Navy, the maximum age allowed to enlist and ship to basic training is 34 years old. In the Naval Reserves, the maximum age is 39 years old.
In the United States Marine Corps, the maximum age allowed to enlist and ship to basic training is 29 years old. In the Marine Corps Reserves, the maximum age is 29 years old.
In the United States Coast Guard, the maximum age allowed to enlist and ship to basic training is 27 years old.¹ In the Coast Guard Reserves, the maximum age is 39 years old.
In the United States Coast Guard, those selected to attend A-school directly upon enlistment may be up to 32 years old; mainly for prior service members.
For prior service members looking to enlist back into the military, they can use their prior service basically as an age deductable when reenlisting. For example, if one has 4 years of credible service in the Army and if they are looking to join the Air Force, but are past the age of 27 but are not more than 31 years old, they can have their prior service years deducted from their age.
Source: Today’s Military