He started out as 13 at first but turned fourteen in the fourth book of the series
Derek turned 14 in the 1st or 2nd book. This is a more accurate answer. I messed up on the other one.
In the book "Stone Cold" by Robert Swindells, Toya is 14 years old. She is described as a runaway teenager who is trying to survive on the streets of London.
He is 22 years old
In life with Derek Lizzie is 11 years old.
As of the 2014 MLB season, Derek Dietrich is 24 years old.
As of the 2014 MLB season, Derek Norris is 25 years old.
Derek Roy is 28 years old (birthdate: May 4, 1983).
Derek Hough is 32 years old (birthdate: May 17, 1985).
Derek Dooley is 49 years old (birthdate: June 10, 1968).
Derek Luke is 43 years old (birthdate: April 24, 1974).
Derek Riddell is 46 years old (birthdate: January 11, 1967).
Derek Fathauer is 25 years old (birthdate: January 20, 1986).