40 years old now in 2025 - what year born?

Today, 19 December 2025 if a person is 40 years old now, what year was he born? See below:

If 40 years old, year of birth is:
1984 or 1985*

*It depends on whether he was born before or after 1 January 1985.

If now 40 years and his year of birth 1984 if he was born after 19 December 1984 until 31 december 1984 inclusively, and his year of birth is 1985 if he was born from 1 January 1985 to 19 December 1985.

Other examples:

If now 7 years, year of birth 2017 or 2018
If now 17 years, year of birth 2007 or 2008
If now 29 years, year of birth 1995 or 1996
If now 81 years, year of birth 1943 or 1944

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