31 years old now in 2025 - what year born?

Today, 5 December 2025 if a person is 31 years old now, what year was he born? See below:

If 31 years old, year of birth is:
1993 or 1994*

*It depends on whether he was born before or after 1 January 1994.

If now 31 years and his year of birth 1993 if he was born after 5 December 1993 until 31 december 1993 inclusively, and his year of birth is 1994 if he was born from 1 January 1994 to 5 December 1994.

Other examples:

If now 5 years, year of birth 2019 or 2020
If now 18 years, year of birth 2006 or 2007
If now 22 years, year of birth 2002 or 2003
If now 47 years, year of birth 1977 or 1978

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