31 years old now in 2023 - what year born?

Today, 1 December 2023 if a person is 31 years old now, what year was he born? See below:

If 31 years old, year of birth is:
1991 or 1992*

*It depends on whether he was born before or after 1 January 1992.

If now 31 years and his year of birth 1991 if he was born after 1 December 1991 until 31 december 1991 inclusively, and his year of birth is 1992 if he was born from 1 January 1992 to 1 December 1992.

Other examples:

If now 12 years, year of birth 2010 or 2011
If now 17 years, year of birth 2005 or 2006
If now 27 years, year of birth 1995 or 1996
If now 70 years, year of birth 1952 or 1953

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