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.