120 years old now in 2023 - what year born?

Today, 30 November 2023 if a person is 120 years old now, what year was he born? See below:

If 120 years old, year of birth is:
1902 or 1903*

*It depends on whether he was born before or after 1 January 1903.

If now 120 years and his year of birth 1902 if he was born after 30 November 1902 until 31 december 1902 inclusively, and his year of birth is 1903 if he was born from 1 January 1903 to 30 November 1903.

Other examples:

If now 5 years, year of birth 2017 or 2018
If now 17 years, year of birth 2005 or 2006
If now 18 years, year of birth 2004 or 2005
If now 31 years, year of birth 1991 or 1992

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