39 years old now in 2024 - what year born?

Today, 12 September 2024 if a person is 39 years old now, what year was he born? See below:

If 39 years old, year of birth is:
1984 or 1985*

*It depends on whether he was born before or after 1 January 1985.

If now 39 years and his year of birth 1984 if he was born after 12 September 1984 until 31 december 1984 inclusively, and his year of birth is 1985 if he was born from 1 January 1985 to 12 September 1985.

Other examples:

If now 9 years, year of birth 2014 or 2015
If now 14 years, year of birth 2009 or 2010
If now 26 years, year of birth 1997 or 1998
If now 68 years, year of birth 1955 or 1956

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