121 years old now in 2023 - what year born?

Today, 31 January 2023 if a person is 121 years old now, what year was he born? See below:

If 121 years old, year of birth is:
1901 or 1902*

*It depends on whether he was born before or after 1 January 1902.

If now 121 years and his year of birth 1901 if he was born after 31 January 1901 until 31 december 1901 inclusively, and his year of birth is 1902 if he was born from 1 January 1902 to 31 January 1902.

Other examples:

If now 9 years, year of birth 2013 or 2014
If now 14 years, year of birth 2008 or 2009
If now 25 years, year of birth 1997 or 1998
If now 81 years, year of birth 1941 or 1942

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