Just heard a terrible statistic (last week) from FOX: 71% of the nations military age "persons" are inegible due to weight, can't pass the test (too stupid), and medically or legally disqualified. And of the remaining 29%, only 4% could be considered eligible for "elite" force assignment.
I suspect this number, 71%, is flipped from VN. I believe most were qualified (disregarding McNamara's Forest Gump experiment), but many ran, or otherwise sought deferment. I know I handed in my draft card in 63 to enter the AF.
During VN, they ran the fat off us. Most wanted to do the best they could on the tests; I suppose some could dummy out, trying to get out of Uncle's service. Medical and legal disqualification (drugs and felons) is usually a result of lifestyle choices: excepting preexisting medical (disease and physical imparement).