39 years old now in 2024 - what year born?

Today, 4 December 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 4 December 1984 until 31 december 1984 inclusively, and his year of birth is 1985 if he was born from 1 January 1985 to 4 December 1985.

Other examples:

If now 12 years, year of birth 2011 or 2012
If now 12 years, year of birth 2011 or 2012
If now 19 years, year of birth 2004 or 2005
If now 60 years, year of birth 1963 or 1964

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