I partially agree with you. Partially because it makes sense for certain stats like stamina, top speed, acceleration. Obviously when you are 40, you are not faster as when you were 20.
Anyways, it's a pain indeed when you see your favorite player loosing stats, or when he stops improving. For example, my Matthews stopped improving at 19, and his stats will decrease soon !!! Crazy Konami !! Stanley Matthews played until the age of 50 !!
For your Schevchenko, have you tried training him in the off-season ?
Because I had R. Ferdinand who lost stats every season but during the off-season he would still improve. Does not make sense at all...