31 years old now in 2022 - what year born?

Today, 6 December 2022 if a person is 31 years old now, what year was he born? See below:

If 31 years old, year of birth is:
1990 or 1991*

*It depends on whether he was born before or after 1 January 1991.

If now 31 years and his year of birth 1990 if he was born after 6 December 1990 until 31 december 1990 inclusively, and his year of birth is 1991 if he was born from 1 January 1991 to 6 December 1991.

Other examples:

If now 6 years, year of birth 2015 or 2016
If now 16 years, year of birth 2005 or 2006
If now 21 years, year of birth 2000 or 2001
If now 65 years, year of birth 1956 or 1957

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