So people often trot out 20 years as a full military career, and in many cases it is what the goal is and has been for decades. But often especially senior officers in command billets will serve significantly longer periods of time. 20 is often talked about because for decades it has been the time required for receiving a pension. Here is a nice breakdown of how the system and particularly retirement pay has evolved since the 80's, and you can see the benefits of staying in to rack up better pay. But there are counterbalanciong regulations, for officers there is an "up or out" system where if you fail to be selected for promotion too many times in a row you are forced out.

And then we come to the mandatory retirement ages. For reference's sake here is where they stand now.

10 U.S. Code § 633: Mandates retirement for O5s not on a promotion list to O6 after 28 years of active commissioned service, except for certain officers in the Navy or Marine Corps who are either limited duty officers or permanent professors at the United States Naval Academy.

10 U.S. Code § 634: Mandates retirement for O6s not on a promotion list to O7 after 30 years of active commissioned service, except for certain officers in the Navy or Marine Corps who are either limited duty officers or permanent professors at the United States Naval Academy.

10 U.S. Code § 635: Mandates retirement for O7s not on a promotion list to O8 after 30 years of active commissioned service or 5 years in grade, whichever is later.

10 U.S. Code § 636: Mandates retirement for O8s after 35 years of active commissioned service or 5 years in grade, whichever is later; after 38 years of active commissioned service for O9s; and after 40 years of active commissioned service for O10s.

10 U.S. Code § 1251: Mandatory retirement age is age 62 for all officers other than general or flag officers. Service Secretaries may defer the retirement of health professional officers and chaplains until the age of 68.

10 U.S. Code § 1252: Mandatory retirement age for permanent professors at service schools is age 64.

10 U.S. Code § 1253: Mandatory retirement age for general and flag officers is age 64. Officers in O9 and O10 positions may have retirement deferred until age 66 by the SECDEF or until age 68 by the President.

10 U.S. Code § 3911, § 6323, and § 8911: Regular and reserve officers in the Armed Forces may retire after 20 years of service, at least 10 of which must have been as an active commissioned officer.

10 U.S. Code § 3918, § 6322, and § 8918: Regular officers in the Armed Forces with at least 30 years of service may be retired upon their request, at the discretion of the President (or the discretion of the Secretary of the Navy for Navy and Marine Corps officers).

From here: http://dopma-ropma.rand.org/retirement-for-years-of-service.html Understandably this would still be contingent on good health and an ability to still pass height/weight regs and a fitness test that does scale for age.

So for instance legendary fighter pilots for the Air Force like Robin Olds(born 1922!) could enter West Point just before WW2, commission as a 2LT after just 3 years thanks to an accelerated war curriculum during the war, then go on to command formations in Vietnam(were he backed up his 12 WW2 kills with 4 more MiG's) in the mid 60's and retire in the early 70's because he had risen sufficiently in rank. A very similar story exists for Chuck Yeager(born 1923), famous for being the first to break the sound barrier, but who had enlisted in the Air Corps in WW2 before commissioning, and would only retire in the mid 70's after a command tour in Vietnam when he flew combat missions as well. Olds' XO in Vietnam then was another similar story who had actually seen combat in Korea, Col. Vermont Garrison, born in 1915 he enlisted in 1941 in the Royal Air Force because of failing out of US flight training before later swapping back to the USAAF. Famous for making Ace in both WW2 and over Korea, Olds actually said he did what he could to keep him away from MiG's and aerial combat in Vietnam as his eyesight wasnt what it once was, but that his value as a bombing leader and pillar of strength and gravitas was invaluable as well.