What year should a person be born to be 21 years old?

The calculation would be Current Year - 21 = Birth Year. To be 21 today, you have to be born on or before today's date of that year. Example: Today is June 27, 2007. 2007 - 21 = 1986. You are at least 21 years old today if you were born on or before June 27, 1986.