In Bionic Birthday Fail, Donald says he is 38.
Two seasons later, in Taken, Douglas shows Leo a video of his 40th birthday party.
Given that Douglas states he is the younger brother, this would suggest that the age difference is extremely slight, no more then two years, if a season is equivalent to one year real time.
Assuming this theory to be correct, Douglas was 36 when the show started and he makes his first appearance (towards the end of season 1).