51 years old now in 2023 - what year born?

Today, 30 November 2023 if a person is 51 years old now, what year was he born? See below:

If 51 years old, year of birth is:
1971 or 1972*

*It depends on whether he was born before or after 1 January 1972.

If now 51 years and his year of birth 1971 if he was born after 30 November 1971 until 31 december 1971 inclusively, and his year of birth is 1972 if he was born from 1 January 1972 to 30 November 1972.

Other examples:

If now 10 years, year of birth 2012 or 2013
If now 14 years, year of birth 2008 or 2009
If now 20 years, year of birth 2002 or 2003
If now 35 years, year of birth 1987 or 1988

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