I don’t know what nurses and lawyers do in the modern world, but Physician is traditionaly a command position. Apart from surgeons, the traditional role of a hospital doctors is to make decisions: somebody else does the action. In the English tradition, surgeons were tradesmen, a distintion marked by the traditional title of “Mr” for surgions, and “Dr” for other doctors. In the modern world, surgeons are doctors before they start surgical traiing, so they are “officers” first.
Regarding SF, old guys don’t recover as fast, but they feel less pain. Athletes peak as sprinters when young, but the distance racers are older, long distance is older still, and ultra-marathon is sometimes people that are ~old~. British army officers are rotated through SF, so that they learn what it’s about. Enlisted roles stay in until retirement.