Players do never retire in Futuball.
When a player is born he has a small number below his name on his player card. This is his "energy".
Energy is amount of match time left to play before reaching critical condition. It is typically 250 competitive matches a player can play from being created.
Friendly matches, world tour matches and ladder matches does not take much energy from a player btw.
When a player reaches 0 energy, his condition will change to "critical". At critical condition a players attributes will drop to 1/4 of what they are normally.
You can still play a player when his condition is critical, but he will not perform very well because his attributes have dropped a lot.
The only way to get more energy for a player is be recharing him in the cryo chargers. But as there is only room for 3 players to be recharged at the time, it might be better to dismantle and sell your player for a few F-coins and instead rebuild a new player on the empty bot you get back when you dismantle your player.
Advanced info
Player energy goes down a bit more in some matches. Here is how much the career energy of player typically drops per 90 mins of a match type played*:
Champions League Match = 1.00
Cup Match = 1.00
League match = 1.00
World Tour Match = 0.20
Ladder Match = 0.05
Friendly match = 0.01
*values are without any boost from club expertize to player energy dropping slower.
So basically this means a player with 250 career energy can play 250 full competitive league, cup and championsleague before reaching 0 career energy.