51 years old now in 2025 - what year born?

Today, 19 December 2025 if a person is 51 years old now, what year was he born? See below:

If 51 years old, year of birth is:
1973 or 1974*

*It depends on whether he was born before or after 1 January 1974.

If now 51 years and his year of birth 1973 if he was born after 19 December 1973 until 31 december 1973 inclusively, and his year of birth is 1974 if he was born from 1 January 1974 to 19 December 1974.

Other examples:

If now 7 years, year of birth 2017 or 2018
If now 16 years, year of birth 2008 or 2009
If now 19 years, year of birth 2005 or 2006
If now 60 years, year of birth 1964 or 1965

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