I think there is a misunderstanding here.
If you stay less than 20 years in military service, you get NO retirement pay, although the time still counts towards federal service time. There is no age requirement if you put in 20 or more years on active duty (have to wait if part of the 20 years is reserves duty). You can start collecting retirement at 37, if you joined at 17, and stayed for 20 years... however, you are still considered a "reservist" (Fleet Reserve for Navy), and CAN be recalled to active duty until you reach 30 years "in".
Retirement pay for 20 years is half of BASE PAY; since a whole lot of a military pay check is due to allowances, this isn't all that much-- some of those vets live on the street because they can't afford anything else. It's something like $400/month (less taxes/FICA, etc) for an E-5 with 20 years.... Retirement pay for 30 years is 3/4 of base pay. Reservist retirement pay depends on how many "days" of active duty time they have served-- any active duty service, the weekend reserve duties and the two weeks/year active service.
If you are a retired "zero", and go to work for the federal govt., you can't collect military retirement pay at the same time, but if you had been enlisted, you can. Kaytee "Simplexities" on