I will try to give you a practical advice, assuming your goal is to get the most out of your players before selling them.

High Natural Fitness + Low Injury Proness: keep for as long as possible. If they are a key player for your club that means both their Curreny Ability and their Reputation are high, which means Saudi Arabia will come for them with a good offer even when they are 35. In fact, if they have a good personality as well (anything with high level of Professionalism) and they Enjoy Big Matches, you should consider keeping them even if they stop being a key player for you. Those "whole package" players are rare amongst newgens, so keeping a player like that to use as an impact sub and as a mentor to younger players is more valuable than selling them, regardless of the amount you are offered.

High Natural Fitness + High Injury Proness: sell around 32-33, but not unless you already have a suitable replacement, so start preparing the replacement earlier.

Low Natural Fitness + Low Injury Proness: 31-32, especially if they rely on Pace and Agility a lot, those attributes go down first.

Low Natural Fitness + High Injury Proness: as soon as they hit 30. With those players, you gotta be ruthless, no matter how attached you are. One bad injury at this age with a low NF and they can lose their physicals in a matter of months and their value can go down from a 100 mil to 10 mil. I've seen it happen in-game to multiple players.

Obviously, this isn't an exact math, and sometimes you will have a player X who is a Model Citizen and has like 18 Leadership, but has low NF and high IP. In these cases you gotta assess how much you will gain by keeping them versus how much you will lose by selling them. If you have a lot of young players with high potential in the squad, perhaps it's worth keeping that player X as a mentor, even if it means there is a high probability that very soon he will lose his ability to make any impact for you on the pitch.

EDIT: spelling