I'm going to look at it from a programming standpoint. If you left the skills ratings exactly the same as they are, then diminished only the physical ratings, I'm guessing you'd end up with older players being just a little too good at doing the things they're good at. A guy with 98 baserunning and, say, 30 speed, might still be capable of stealing 30 or so. JUST A GUESS. You'd have certain players playing well into their 40s.
So in that case, you make the skills ratings decline "artificially" too, and then you get a 38 year old who's properly gone to pot.