115 years old now in 2025 - what year born?

Today, 19 December 2025 if a person is 115 years old now, what year was he born? See below:

If 115 years old, year of birth is:
1909 or 1910*

*It depends on whether he was born before or after 1 January 1910.

If now 115 years and his year of birth 1909 if he was born after 19 December 1909 until 31 december 1909 inclusively, and his year of birth is 1910 if he was born from 1 January 1910 to 19 December 1910.

Other examples:

If now 6 years, year of birth 2018 or 2019
If now 12 years, year of birth 2012 or 2013
If now 21 years, year of birth 2003 or 2004
If now 90 years, year of birth 1934 or 1935

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