Previous research: numerous different Google searches on retirement, mandatory retirement, US Army etc as well as digging through several US armed forces websites. Military.com's Army Professional Developement Center was very helpful, as was Wikipedia's information on the US Army.
Subject: The subject is a retired ex-Special Forces soldier from the U.S. army, specifically a career NCO that spent most of his career assigned to the 1st Special Forces Group, which is based in Okinawa and is responsible for the Pacific Rim, and his career extended from Vietnam to the first Gulf War (1960-1991). Warning: I'm throwing around years and timelines here, trying to see what is within the realm of general plausibility.
The Question, in a rambling sort of way:
For a man that served out his full 30 years (plus a one year extension on his mandatory retirement due to Operations Desert Shield and Desert Storm), what would be the most plausible rank for the soldier to hold? I was shooting for either First Sergeant (1SG, E-8) or maybe even Master Sergeant (MSG, E-8), but I am not sure this is reasonable, given that the man in question is quite competent. Does MSG stretch credulity a bit? Would he have been more likely required to advance up to an E-9 rank like Sergeant Major (SGM)? If he made SGM or CSM (Command Sergeant Major), then obtaining the rank of E-5 in 1963 would be fine, though potentially much later would work too of course. For a final rank of E-8, he could have made Sergeant no earlier than 1968 if my figuring is correct, which would mean it took him 8 years in the service to reach the rank of Sergeant.
Now, that last is a big part of the question: is it plausible for a man to be in the Special Forces for 8 years before reaching the rank of E-5?
If not, then the character will have to be an E-9 in the time period that I am concerned about. If it is, then E-8 is definitely still an option.