21 years old now in 2025 - what year born?

Today, 19 December 2025 if a person is 21 years old now, what year was he born? See below:

If 21 years old, year of birth is:
2003 or 2004*

*It depends on whether he was born before or after 1 January 2004.

If now 21 years and his year of birth 2003 if he was born after 19 December 2003 until 31 december 2003 inclusively, and his year of birth is 2004 if he was born from 1 January 2004 to 19 December 2004.

Other examples:

If now 12 years, year of birth 2012 or 2013
If now 14 years, year of birth 2010 or 2011
If now 20 years, year of birth 2004 or 2005
If now 39 years, year of birth 1985 or 1986

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