21 years old now in 2023 - what year born?

Today, 1 December 2023 if a person is 21 years old now, what year was he born? See below:

If 21 years old, year of birth is:
2001 or 2002*

*It depends on whether he was born before or after 1 January 2002.

If now 21 years and his year of birth 2001 if he was born after 1 December 2001 until 31 december 2001 inclusively, and his year of birth is 2002 if he was born from 1 January 2002 to 1 December 2002.

Other examples:

If now 7 years, year of birth 2015 or 2016
If now 18 years, year of birth 2004 or 2005
If now 19 years, year of birth 2003 or 2004
If now 37 years, year of birth 1985 or 1986

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