The military has up or out requirements. If a member does not get promoted within certain time requirements, he is separated. He can't be separated for failure of promotion, then reenlist and start over.
Then there are age and fitness requirements. (I was granted a commission at 31, as old as you could be without some special waiver. I met all fitness standards, but going through my basic training with a bunch of early 20 somethings was a little challenging.)
Being out for 2-5 years is one thing, but 15 years is another. I suspect this guy was granted a waiver on the age limits. He most likely met all fitness (and weight) requirements, or wouldn't have sought a reenlistment.
Many who separate (but not 'for cause') do so with a period of active or inactive reserve commitment following separation. Historically, when a need for more manpower comes along (war), those people can be recalled to active duty. (I had a 17+yr inactive reserve commitment at separation. They forgot about me and didn't issue me my honorable discharge until over 20 years. They even apologized. I wondered what had happened, but didn't think I'd get recalled to active duty near 60.

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