42 years old now in 2021 - what year born?

Today, 18 January 2021 if a person is 42 years old now, what year was he born? See below:

If 42 years old, year of birth is:
1978 or 1979*

*It depends on whether he was born before or after 1 January 1979.

If now 42 years and his year of birth 1978 if he was born after 18 January 1978 until 31 december 1978 inclusively, and his year of birth is 1979 if he was born from 1 January 1979 to 18 January 1979.

Other examples:

If now 6 years, year of birth 2014 or 2015
If now 13 years, year of birth 2007 or 2008
If now 27 years, year of birth 1993 or 1994
If now 42 years, year of birth 1978 or 1979

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