39 years old now in 2023 - what year born?

Today, 26 March 2023 if a person is 39 years old now, what year was he born? See below:

If 39 years old, year of birth is:
1983 or 1984*

*It depends on whether he was born before or after 1 January 1984.

If now 39 years and his year of birth 1983 if he was born after 26 March 1983 until 31 december 1983 inclusively, and his year of birth is 1984 if he was born from 1 January 1984 to 26 March 1984.

Other examples:

If now 11 years, year of birth 2011 or 2012
If now 13 years, year of birth 2009 or 2010
If now 26 years, year of birth 1996 or 1997
If now 47 years, year of birth 1975 or 1976

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