20 years old now in 2023 - what year born?

Today, 1 December 2023 if a person is 20 years old now, what year was he born? See below:

If 20 years old, year of birth is:
2002 or 2003*

*It depends on whether he was born before or after 1 January 2003.

If now 20 years and his year of birth 2002 if he was born after 1 December 2002 until 31 december 2002 inclusively, and his year of birth is 2003 if he was born from 1 January 2003 to 1 December 2003.

Other examples:

If now 8 years, year of birth 2014 or 2015
If now 14 years, year of birth 2008 or 2009
If now 19 years, year of birth 2003 or 2004
If now 88 years, year of birth 1934 or 1935

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