41 years old now in 2021 - what year born?

Today, 12 September 2021 if a person is 41 years old now, what year was he born? See below:

If 41 years old, year of birth is:
1979 or 1980*

*It depends on whether he was born before or after 1 January 1980.

If now 41 years and his year of birth 1979 if he was born after 12 September 1979 until 31 december 1979 inclusively, and his year of birth is 1980 if he was born from 1 January 1980 to 12 September 1980.

Other examples:

If now 11 years, year of birth 2009 or 2010
If now 17 years, year of birth 2003 or 2004
If now 21 years, year of birth 1999 or 2000
If now 65 years, year of birth 1955 or 1956

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