19 years old now in 2023 - what year born?

Today, 1 December 2023 if a person is 19 years old now, what year was he born? See below:

If 19 years old, year of birth is:
2003 or 2004*

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

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

Other examples:

If now 7 years, year of birth 2015 or 2016
If now 15 years, year of birth 2007 or 2008
If now 23 years, year of birth 1999 or 2000
If now 79 years, year of birth 1943 or 1944

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