The age displayed is actually AGE GROUP, and not Age.

Partly for privacy reasons and partly that Age Group is widely used for categorising players.

The AGE GROUP is calculated as the CURRENT YEAR minus YEAR OF BIRTH.

Eg. For a player born in 2010 during the year 2020.

2020 - 2010 = Under 10. 

During the entire 2020 calendar year, players born in 2010 will be able to participate in Under 10 age divisions, regardless of if they were born on Jan 1 and are actually 10 years old, or born on Dec 31 and are 9 years of age.

This is the usual regulation for allocating players to age-divisions right from FIDE through to local club events.