The only system I can think of that even sort of models characters 'getting rusty' is Classic Traveller. (I heard that the First Edition Exalted Lunar book's Storyteller chapter says that you'd get weak and flabby if you stay in cities to long, but I don't believe that it gives rules for this.) Classic Traveller characters in creation only get a choice on the sort of skills they want, not on exactly which they get, and once created it has a funky four year period system for determining how skills are built up. If I remember correctly, characters which abortedabort the process, lostlose a point in the skill they were working on.
(If you're interested, I could dig up the rules later.)