If you are 40 years old when was you born?

This answer will change with the years. To be accurate, take the year and subtract 40 from it.

So:

2008 - 40 = 1968

2009 - 40 = 1969

2010 - 40 = 1970

2011 - 40 = 1971

2012 - 40 = 1972

2013 - 40 = 1973

2014 - 40 = 1974

2015 - 40 = 1975

2016 - 40 = 1976