Basil Plumley was a real person (born in 1920, so he would have been in his forties during the battle covered in Joe Galloway's and Harold G. Moore's book describing the battle, which was adapted into the movie that you are referring to).

If Haney was referred to as "gunny," then he wasn't a sergeant. He was a gunnery sergeant, which is its own rank (two ranks above sergeant). In the Army, NCO's whose rank includes the word sergeant get referred to with just that word (except for first sergeant or sergeant major), but the tradition in the Marine Corps (which the rank of gunnery sergeant is unique to) is to either say the whole rank or use a distinctive abbreviation. In this case, "gunny" is the distinctive abbreviation of gunnery sergeant.

The estimates that I've heard for minimum time that it takes to reach gunnery sergeant and sergeant major are 12-15 years and 20-25 years, respectively. For that reason, I would guess that those were just actors who are older than their characters should be.

Senior NCO's with ranks such as command sergeant major, as well as the United States concept of warrant officer (which is distinct from everybody else's concept, every other military considers senior NCO's and warrant officer to be the same think, while in the US, warrant officers are a technical experts whose job is to know absolutely everything about one specific thing) often have decades of service.

It is far from unheard of for a US warrant officer to retire with more than 40 years of service.

However, in most case, these are career soldiers who have been promoted high enough that their duties keep them far behind the front lines, with the exception of a U.S. Army helicopter pilot (helicopter pilots are the most common type of warrant officer).

For non-career soldiers, the only thing that I can think of is the Volkssturm. In 1944, Germany realized that they were losing the war and running out of manpower, so they started conscripting people who were both too old and too young for ordinary conscription.