I'd like to suggest that, when players age, their Intelligence should have a chance to go up. More specifically, players who have played a lot. A 3rd string QB who has played 100 snaps in 15 years probably hasn't had a chance to learn much from playing. But, a safety that has started for 10 straight seasons knows a thing or three more than he did when he started in the league. Considering that intelligence is more like 'football knowledge' than 'ability to do quantum mechanics calculations', it would only make sense that, while players bodies and physical abilities are breaking down, the knowledge they have gathered is growing. I understand that, in this game, skill is similar to what I'm talking about. however, if you look into the game's mechanics, and understand how they (skill and intelligence) each affect a player's playing ability, I think you might agree that intelligence, like skill, should grow with playing time. An example I'd use is two defensive tackles, both the same age (31) and years experience (10), and both 100 skill. Player A has started 160 games (every game since he came into the league). Player B has 16 starts and has only appeared in 93 games. Skill level doesn't show the difference in what these two players have learned from playing the game. intelligence could via growth through experience. Both are in against a draw play. One has seen hundreds and hundreds of draws in real time action over his career, one has not. Player A should have had a chance to improve his ability to recognize a play, and intelligence is a key stat in this area.
To add to this, I personally think Intelligence should 'age' through playing time, not due to actual age. So, whenever the game does calculations to determine if a player loses skills due to aging, it should, at that time, determine if a player is gaining intelligence due to experience. However, the aging process works in the game, Intelligence should increase due to aging, unlike strength, speed, etc.