You must be between the the ages of 17 and 35 to join the U.S. Army. If you are under age 18, you must obtain the consent of your parent or legal guardian.
However, there are exceptions to age restrictions for several different groups. Here are some of those exceptions.
OFFICER CANDIDATE SCHOOL AGE REQUIREMENTS
In order to qualify for Officer Candidate School as an active duty, enlisted Soldier, you must be between 19 and 32 years old (you must enter active duty or ship to training by your 33rd birthday and accept commission prior to age 34).
In order to qualify for Officer Candidate School as a civilian, you must be between 19 and 32 years old (you must enter active duty or ship to training by your 33rd birthday and accept commission prior to age 34).
In order to qualify for Officer Candidate School as a member of the U.S. Army Reserve, you must be between 18 and 32 years old (you must enter active duty or ship to training by your 33rd birthday and accept commission prior to age 34).
CHAPLAIN AGE REQUIREMENTS
You must be at least 21 years of age, but younger than 42 at time of Regular Army commissioning or less than 45 at time of Army Reserve commissioning. Prior service applicants with at least 3 years of prior AFS or creditable USAR service must be younger than 47 at time of Army Reserve commissioning.
JAG CORPS AGE REQUIREMENTS
Applicants must be able to serve 20 years of active commissioned service before reaching the age of 62. Applicants must be under the age of 42 at the time of entry into active duty service.
ACTIVE GUARD RESERVE AGE REQUIREMENTS
For an enlisted Soldier you must be older than 18 years, but younger than 55. If you are a Specialist or a Master Sergeant, you must have less than 15 years of active Federal Service.
You can also visit here to see the age requirements for other branches of the military.