ovo piše u FIFInim pravilima
regulations status and transfer  pdf

Players under the age of 18 may not sign a professional contract for a
term longer than three years. Any clause referring to a longer period shall not
be recognised.

International transfers of players are only permitted if the player is over the
age of 18.
2. The following three exceptions to this rule apply:
a) The player’s parents move to the country in which the new club is
located for reasons not linked to football.
b) The transfer takes place within the territory of the European Union
(EU) or European Economic Area (EEA) and the player is aged between
16 and 18.