You'll only lose ability if you class them later than 40 for fighter, and the respective level for bomber.
It's actually a good idea for future fighter and bomber pilots to skip the "rookie" (scout) classing and leave them "special forces" until they reach their fighter or bomber classing: You don't lose any fighter or bomber ability AND you gain more repair and restore ability.
OK, only 30 displayed ability per sailor for a fighter and a little more for bomber pilots, but with 8 pilots at lvl 120 with all their vets and experts, that adds up quite nicely...