This question may apply to other leagues in other places, but recently I saw an article about Radamel Falcao playing for the Manchester United U-21 team and Rafael doing the same thing. Both players are over 21 and make semi-regular starts for the Manchester United senior squad. How are they also allowed to play for the U-21 reserve team when they're obviously over-age?
Because the rules say that they can:
Clubs are only allowed to field three outfield players over the age of 21, plus the goalkeeper is also permitted to be over 21.
The rules allow for up to three overage outfield players as well as the goalkeeper to be over the age of 21. Usually these players are struggling players whom the manager wants to get playing time or they are players coming back from injury and the manager wants to know if they are game ready.