120 years old now in 2025 - what year born?

Today, 19 December 2025 if a person is 120 years old now, what year was he born? See below:

If 120 years old, year of birth is:
1904 or 1905*

*It depends on whether he was born before or after 1 January 1905.

If now 120 years and his year of birth 1904 if he was born after 19 December 1904 until 31 december 1904 inclusively, and his year of birth is 1905 if he was born from 1 January 1905 to 19 December 1905.

Other examples:

If now 5 years, year of birth 2019 or 2020
If now 17 years, year of birth 2007 or 2008
If now 20 years, year of birth 2004 or 2005
If now 39 years, year of birth 1985 or 1986

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