54 years old now in 2021 - what year born?

Today, 17 September 2021 if a person is 54 years old now, what year was he born? See below:

If 54 years old, year of birth is:
1966 or 1967*

*It depends on whether he was born before or after 1 January 1967.

If now 54 years and his year of birth 1966 if he was born after 17 September 1966 until 31 december 1966 inclusively, and his year of birth is 1967 if he was born from 1 January 1967 to 17 September 1967.

Other examples:

If now 6 years, year of birth 2014 or 2015
If now 13 years, year of birth 2007 or 2008
If now 19 years, year of birth 2001 or 2002
If now 34 years, year of birth 1986 or 1987

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