The original method to deduce a Korean's age included the gestation period for all babies, meaning they were all counted as one-year-olds at birth.
Citizens would also become a year older on January 1, even those who were born in December.
However a law passed in December after strong campaigning fro newly-elected Korean president Yoon Suk Yeol, with the new rules coming into force this week.
For Son, while his age will change in his homeland of South Korea, nothing will be different in the UK.
Born on July 8, 1992, the forward has always been classified by the international method since moving to England and therefore will remain as a 30-year-old.
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Tottenham's registration of the player will also not change, with the club only needing to provide an accurate date of birth as opposed to an age.
As well as a change in age in South Korea, Son has spent his summer playing down links with a lucrative move to Saudi Arabia.
Committing himself to Tottenham, Son said: "I have many things to do in the Premier League.
"Money doesn't matter to me now, and the pride of playing football, to play in my favourite league is important."