121 years old now in 2023 - what year born?

Today, 7 December 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 7 December 1901 until 31 december 1901 inclusively, and his year of birth is 1902 if he was born from 1 January 1902 to 7 December 1902.

Other examples:

If now 4 years, year of birth 2018 or 2019
If now 18 years, year of birth 2004 or 2005
If now 28 years, year of birth 1994 or 1995
If now 63 years, year of birth 1959 or 1960

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