84 years old now in 2024 - what year born?

Today, 15 October 2024 if a person is 84 years old now, what year was he born? See below:

If 84 years old, year of birth is:
1939 or 1940*

*It depends on whether he was born before or after 1 January 1940.

If now 84 years and his year of birth 1939 if he was born after 15 October 1939 until 31 december 1939 inclusively, and his year of birth is 1940 if he was born from 1 January 1940 to 15 October 1940.

Other examples:

If now 7 years, year of birth 2016 or 2017
If now 15 years, year of birth 2008 or 2009
If now 28 years, year of birth 1995 or 1996
If now 90 years, year of birth 1933 or 1934

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