The players are divided into age groups: 18-21, 22-25, 26-29, 30-33, 34.
The only difference is their speed in acquiring experience from games and training.

That's why I'll always favour very young players, as they last longer. An 18-year-old can stay on my squad for four months because they gain status points fast enough to overcome the star losses after each promotion. But once he hits 22 years old, he can no longer keep it up, because the speed of gaining status points is pretty much halved.

Other than that, I don't see observable difference in actual game performances that is caused solely by age. If the skill points are exactly the same, a 30-year-old would perform the same as an 18-year-old, subject to the RNG.

To me, age is about planning for the future, not so much this season. If I buy a 25-year-old, I know I can only keep him for at most two seasons before I have to spend another token to buy a replacement. Now, of course, younger players could cost more tokens to buy at the auction because other people would also bid on them, which could make it cost more.

And this kind of managerial decisions is what makes this game fun.