18 years old now in 2023 - what year born?

Today, 7 December 2023 if a person is 18 years old now, what year was he born? See below:

If 18 years old, year of birth is:
2004 or 2005*

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

If now 18 years and his year of birth 2004 if he was born after 7 December 2004 until 31 december 2004 inclusively, and his year of birth is 2005 if he was born from 1 January 2005 to 7 December 2005.

Other examples:

If now 8 years, year of birth 2014 or 2015
If now 15 years, year of birth 2007 or 2008
If now 24 years, year of birth 1998 or 1999
If now 31 years, year of birth 1991 or 1992

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