The youth movement is on for the MLS' Philadelphia Union.

The soccer club signed 14-year-old prodigy Cavan Sullivan on Thursday in a historic deal.

Although the conditions were not made public, the pact is said to be the largest homegrown deal in league history.

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According to the league's site, "a club may sign a player to a contract without subjecting him to the MLS SuperDraft if the player has been a member of that club’s youth academy for at least one year and has met the necessary requirements. Players joining MLS through this mechanism are known as Homegrown Players."

The contract allows the Union to continue to develop Sullivan and benefit in the short term from his ability, before profiting when he likely heads overseas to the Premier League. It also allows him to stay at home for a few more years.

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Sullivan (14 years, 224 days) is the fifth-youngest player to sign a first-team contract in MLS history. If he plays for the Union before July 29, he'll become the youngest player ever to appear in a match. Freddy Adu was 14 years, 306 days old when he debuted for D.C. United in 2004.

Sullivan is considered one of the best young American players. In a match last April between the U.S. under-15 team and England in Spain, he scored both goals in a 2-2 draw that the Americans won in a penalty shootout.

He was named the best player at the CONCACAF under-15 championships. He had a pair of assists in the 4-2 victory over Mexico in the final.

The deal will also send Sullivan to Manchester City once he turns 18.

The Associated Press contributed to this report.

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