Naturally, when you don't practice at something for a long time, your skill at it will deteriorate.
Why not do that in a game?
After reaching a 'Maximum Skill', earning skill levels will degrade the oldest skill levels you earned.
Dependant on several variables, such as age, your Maximum Skill will change.
In a similar manner, not using skills of a particular stat will cause that stat to fall.
You can offset this by obtaining so high a skill level in one skill that that skill will deteriorate slower.
(Instead of losing a whole skill level like normal, you'ld lose half or smaller increments of experience off of that skill instead)
This prevents players from making god-characters.
(Characters with all skills or stats maxed out)