91 years old now in 2023 - what year born?

Today, 30 November 2023 if a person is 91 years old now, what year was he born? See below:

If 91 years old, year of birth is:
1931 or 1932*

*It depends on whether he was born before or after 1 January 1932.

If now 91 years and his year of birth 1931 if he was born after 30 November 1931 until 31 december 1931 inclusively, and his year of birth is 1932 if he was born from 1 January 1932 to 30 November 1932.

Other examples:

If now 5 years, year of birth 2017 or 2018
If now 15 years, year of birth 2007 or 2008
If now 21 years, year of birth 2001 or 2002
If now 31 years, year of birth 1991 or 1992

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