84 years old now in 2023 - what year born?

Today, 23 September 2023 if a person is 84 years old now, what year was he born? See below:

If 84 years old, year of birth is:
1938 or 1939*

*It depends on whether he was born before or after 1 January 1939.

If now 84 years and his year of birth 1938 if he was born after 23 September 1938 until 31 december 1938 inclusively, and his year of birth is 1939 if he was born from 1 January 1939 to 23 September 1939.

Other examples:

If now 8 years, year of birth 2014 or 2015
If now 16 years, year of birth 2006 or 2007
If now 27 years, year of birth 1995 or 1996
If now 79 years, year of birth 1943 or 1944

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