138 years old now in 2023 - what year born?

Today, 26 September 2023 if a person is 138 years old now, what year was he born? See below:

If 138 years old, year of birth is:
1884 or 1885*

*It depends on whether he was born before or after 1 January 1885.

If now 138 years and his year of birth 1884 if he was born after 26 September 1884 until 31 december 1884 inclusively, and his year of birth is 1885 if he was born from 1 January 1885 to 26 September 1885.

Other examples:

If now 6 years, year of birth 2016 or 2017
If now 15 years, year of birth 2007 or 2008
If now 23 years, year of birth 1999 or 2000
If now 75 years, year of birth 1947 or 1948

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