3 years old now in 2024 - what year born?

Today, 4 December 2024 if a person is 3 years old now, what year was he born? See below:

If 3 years old, year of birth is:
2020 or 2021*

*It depends on whether he was born before or after 1 January 2021.

If now 3 years and his year of birth 2020 if he was born after 4 December 2020 until 31 december 2020 inclusively, and his year of birth is 2021 if he was born from 1 January 2021 to 4 December 2021.

Other examples:

If now 10 years, year of birth 2013 or 2014
If now 15 years, year of birth 2008 or 2009
If now 29 years, year of birth 1994 or 1995
If now 40 years, year of birth 1983 or 1984

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