Do any of the static attributes - Speed, Acceleration, Intelligence, Discipline, Arm Strength, and Release - ever improve?

I have never seen a definitive answer. But I'm almost certain that the answer is no.

No, in fact as a player ages they will begin to decline.

I can understand the decline of physical attributes. But does intelligence decline too? Or discipline? Thanks.

See I would think discipline and intelligence should be things that can improve over time.

Some items like intelligence and discipline do not change. There are actually 3 categories of attributes - attributes that can improve (and will decline as the player ages), attributes that are static until they decline as the player ages, and attributes that are static.

Discipline and intelligence are static - intelligence, for example, doesn't change - a player doesn't actually get smarter, but he does learn more. Intelligence basically drives how fast he learns. Discipline I went back and forth on but settled on keeping it static (otherwise you'll get the same situation we already have with player weight where all players will migrate toward the same level of discipline)

Thank you for clarifying that. Great site you are running here.

If I'm reading this right in terms of other posts we have had about intelligence on the forums, this means that intelligence helps a player achieve his position experience and playbook knowledge faster, but does not directly influence a play on the field. Is that correct?

His playbook knowledge and position experience do play a role on the field. Mostly with decision making attributes, i.e. a LB will be quicker to the hole with higher experience on a running play than one with identical attributes but less experience.

So basically

high intelligence=quicker reactions over time

If that's the case then that makes sense.

The getting started page also says it affects decisions like when to extend routes and stuff .

Yes, items like that also take experience into account.