Maximum service is 30 years for enlisted and there are limits for how long you can stay depending on paygrade.
Most of the guys staying twenty years have kids in high school and have already moved families around ten times or more like gypsies. Sometimes they get to the point that they have weigh family stability versus staying for another hitch. It can be alot of stress on a family.
Some of these jobs are a young man's game too!
What I meant was if the manditory retirement age is at 65 and they accept up to age 45, a person could retire in 20 years at age 65, and have a pension. I know quite a few "geezers" (over age 35) who could run circles around most 18 year olds.