In this episode Don and his mentor talk about first working together and how Don was a green 25 year old. In the final episode of the series Cause and Effect (2010) it is said that Don has been an FBI agent for 15 years. However his birth date is 15th July 1967 so that would make him 28 years old when he started, or he's actually worked 18 years at the FBI by the final episode.