In 2001, FIFA, UEFA and various other European stakeholders developed Regulations for the Status and Transfer of Players. The main rule is that international transfers are only allowed for non-minor players. (Minor players are defined as players under the age of eighteen)
There are six exceptions to this:
1) The child can play for a club in a new country if the child’s parents moved to that country for reasons that were not related to football. (art. 19 par. 2a)
2) This applies to players over the age of 16, and allows them to move within the European Union and/or the European Economic Area. (art. 10 par. 2b)
3) The player and the new club are located in countries that border each other. The clubs must each be within 50km of their shared border. Also, the distance between the two clubs cannot be more than 100km (art. 19, par 2c)
4) The player’s move is based on humanitarian reasons and he is moving without his parents. (art. 19 par. 2d)
5) The player is participating in an academic program abroad as a student (art. 19, par. 2e)
6) After a player has been living continuously in a country for a period of at least five years, he may register for the first time with an association. (art.19, par 3 and 4)
There is a special subcommittee appointed by the Player’s Status committee that looks at each minor player’s application individually. The submission and handling of these applications is managed completely by the association that is wanting to register the minor player through the TMS (transfer matching system.)
FIFA has an updated guide that contains an overview of the application process, a complete description of the documents that are required for a minor application and a FAQ section. You can access that here.
Prior to these protections being put into place, there had been instances in which young players had been brought over to join Academy clubs and then dropped when they did not perform to the expected standards. In many cases, the child was left with no place to stay, no funds and no way of returning to their home country. These rules were created in response to these situations in an effort to ensure that minors are protected and not abused.
Although these rules can seen strict and prohibitive to the many foreign players who want to become a professional soccer player and hope to better their chances by training abroad, they were created to protect minor children.