4 years old now in 2023 - what year born?

Today, 23 September 2023 if a person is 4 years old now, what year was he born? See below:

If 4 years old, year of birth is:
2018 or 2019*

*It depends on whether he was born before or after 1 January 2019.

If now 4 years and his year of birth 2018 if he was born after 23 September 2018 until 31 december 2018 inclusively, and his year of birth is 2019 if he was born from 1 January 2019 to 23 September 2019.

Other examples:

If now 3 years, year of birth 2019 or 2020
If now 15 years, year of birth 2007 or 2008
If now 29 years, year of birth 1993 or 1994
If now 50 years, year of birth 1972 or 1973

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