Found the rules on the English FA website
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The new FIFA Statutes which came into force on 1 January 2004 saw a significant change regarding international eligibility for players with dual nationality.
The new Article 15 of the Regulations Governing the Application of the FIFA Statutes has been amended to state that that "a player may exercise his right to change Associations only if he has not played at "A" international level for his country and if, at the time of his first full or partial appearance in an international match in an official competition of any other category, he already had such nationalities."
Previously, players with dual nationality were deemed to have committed themselves to one national association after playing a competitive match at any level.
The new regulation also states that up to their 21st birthday, a player with dual nationality, a player may only once request changing the association for which he is eligible to play international matches.
However, any player who has already had their 21st birthday at the time of these provisions being implemented and who hold dual nationality have twelve months in which to request to change Associations.
This request must be submitted in writing to FIFA and will be decided upon by the FIFA Players' Status Committee.
After making the request the player is no longer qualified to play for his current national association.
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The way I read this is that any player over 21 who has played for a country at underage level but not at senior level has until the end of this year to change country. Anyone under 21 who has played underage football but hasn't played for the senior team can change up until their 21st birthday.
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