40 years old now in 2024 - what year born?

Today, 10 October 2024 if a person is 40 years old now, what year was he born? See below:

If 40 years old, year of birth is:
1983 or 1984*

*It depends on whether he was born before or after 1 January 1984.

If now 40 years and his year of birth 1983 if he was born after 10 October 1983 until 31 december 1983 inclusively, and his year of birth is 1984 if he was born from 1 January 1984 to 10 October 1984.

Other examples:

If now 6 years, year of birth 2017 or 2018
If now 12 years, year of birth 2011 or 2012
If now 28 years, year of birth 1995 or 1996
If now 97 years, year of birth 1926 or 1927

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