What law is it that allows you to retire after 17 years in the military?

I’ve seen people retire after 17 years due to things like PRT failures and they always say it’s a law that you can’t get kicked out unless you do something illegal once you hit 17. It makes sense to me because if the military could get away with it they would kick everyone out after 19 and 364 days to save money. Can someone please give me the law or instruction number? I can’t find it anywhere.