21 years old now in 2023 - what year born?

Today, 22 March 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 22 March 2001 until 31 december 2001 inclusively, and his year of birth is 2002 if he was born from 1 January 2002 to 22 March 2002.

Other examples:

If now 10 years, year of birth 2012 or 2013
If now 15 years, year of birth 2007 or 2008
If now 23 years, year of birth 1999 or 2000
If now 39 years, year of birth 1983 or 1984

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