21 years old now in 2026 - what year born?

Today, 28 October 2026 if a person is 21 years old now, what year was he born? See below:

If 21 years old, year of birth is:
2004 or 2005*

*It depends on whether he was born before or after 1 January 2005.

If now 21 years and his year of birth 2004 if he was born after 28 October 2004 until 31 december 2004 inclusively, and his year of birth is 2005 if he was born from 1 January 2005 to 28 October 2005.

Other examples:

If now 11 years, year of birth 2014 or 2015
If now 17 years, year of birth 2008 or 2009
If now 22 years, year of birth 2003 or 2004
If now 57 years, year of birth 1968 or 1969

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