6 years old now in 2025 - what year born?

Today, 19 December 2025 if a person is 6 years old now, what year was he born? See below:

If 6 years old, year of birth is:
2018 or 2019*

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

If now 6 years and his year of birth 2018 if he was born after 19 December 2018 until 31 december 2018 inclusively, and his year of birth is 2019 if he was born from 1 January 2019 to 19 December 2019.

Other examples:

If now 9 years, year of birth 2015 or 2016
If now 15 years, year of birth 2009 or 2010
If now 21 years, year of birth 2003 or 2004
If now 98 years, year of birth 1926 or 1927

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