Some [link|http://www.sss.gov/FSpostdefer.htm|Selective Service] rules.
During the Vietnam War era, (I think in early 1962) I was classified 1-A and had a "letter from the president" and indeed had a physical at the Boston Army Base. When I told my supervisor (a professor) about it, he quickly got me an "occupational" deferment for working on The Office of Naval Research (ONR) and National Science Foundation (NSF) sponsored project. Within a couple years, I was the father of two kids so it became a different kind of deferment. In theory, I was "on the hook" till age 45. But you know they want those malleable 18-24 year olds. It was not a planned deal on my part, but that's how it worked out.
My "service" was a year of Air Force ROTC while I was a freshman in college.
My brother, on the other hand, spent 27 years in the Air Force.