54 years old now in 2025 - what year born?

Today, 19 December 2025 if a person is 54 years old now, what year was he born? See below:

If 54 years old, year of birth is:
1970 or 1971*

*It depends on whether he was born before or after 1 January 1971.

If now 54 years and his year of birth 1970 if he was born after 19 December 1970 until 31 december 1970 inclusively, and his year of birth is 1971 if he was born from 1 January 1971 to 19 December 1971.

Other examples:

If now 5 years, year of birth 2019 or 2020
If now 16 years, year of birth 2008 or 2009
If now 23 years, year of birth 2001 or 2002
If now 42 years, year of birth 1982 or 1983

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