40 years old now in 2025 - what year born?

Today, 24 April 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 24 April 1984 until 31 december 1984 inclusively, and his year of birth is 1985 if he was born from 1 January 1985 to 24 April 1985.

Other examples:

If now 11 years, year of birth 2013 or 2014
If now 15 years, year of birth 2009 or 2010
If now 30 years, year of birth 1994 or 1995
If now 57 years, year of birth 1967 or 1968

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