As in real life, players grow older in F4Football too. Age of a player progresses with every season in the game. The lowest possible age for a player is 16 and the highest can go upto 40.
Effects of Age
Players tend to train slower as they grow older. So it is best to develop skills of a player while they are still young. They might also lose a bit of Stamina and Speed towards the end of their careers.
The player's value will also drop as their age increases.
Players retire when they are old, and there are various factors that affect their retirement. At some point in their career, they might decide that they are too old to play professional football. They hang up their boots if they sustain a major recurring injury or even if they think they are not good enough to fit in their current team's plans and don't have a future anywhere else.
Players who decide to retire will announce that at the start of a season. So you can still play them in that whole season, and look for replacements or make alternative plans accordingly.