There are multiple "sergeant" ranks. E-5 through E-9, with increasing responsibility along the way.
In the Army:
E-5 - Sergeant: typically runs a 3-4 member combat team or section
E-6 - Staff Sergeant: Will run squad or larger section, like a company-level supply section, handling logistics for 200 or so troops.
E-7 - Sergeant First Class: NCO in charge of a platoon (50-ish troops), or can act as the First Sergeant, the senior NCO in a company-sized unit.
E-8 - Master Sergeant; First Sergeant; Sergeant Major. A Master Sergeant is a staff position, usually at the battalion level or higher (5 or so companies). They're the NCOs running Supply, Operations, Security and so forth for the battalion. A First Sergeant is the senior NCO in a company. Sergeant Major is the similar position for a battalion, brigade or similar sized command.
E-9 - Sergeant Major. This is a command-level senior NCO. Lots of variants.
https://en.wikipedia.org/wiki/United_States_Army_enlisted_rank_insignia is a good explanation.
20 years after making E5, a person is likely to be retired. They might have topped out at E8 is they were particularly motivated, and had combat experience. Certain MOSs (Military Occupational Specialties - your job field) have their NCOs advance more rapidly than others. It's entirely possible for NCOs in some career fields to spend 6-7 years at E6.