Every season, he lost 20% to all stats. Gray abilities have a penalty, so the older the player, the less he learn... in this case, I try to show it (it'a just an example):
age grayskills
18 y.o. 100%
19 y.o. 80%
20 y.o. 60%
21 y.o. 40%
22 y.o. 20%
23 y.o. 1%
24 y.o. 1%
.
.
.learned keyattributes
+25%
+25%
+25%
+20%
+20%
+15%
+15%
.
.
.keyattributes at age
125%
130%
135%
135%
135%
130%
130%
.
.
.
The keyattributes raise very fast in the early days. If the player learn something after the game, it's everytime keyattributes (can't remember, they learned a gray skills).