When he was 5, as revealed in a flashback of how he first met his future frenemy Mr. Baumann, Ben wore green overalls with a white shirt underneath it. White numbers saying "5 1/2" where on a black square in front of his overalls.
In the original series and Omniverse flashbacks, Ben's usual clothes consists of a black-striped, white shirt with short sleeves, green pants and black and white shoes with black stripes. Ben wears the prototype Omnitrix on his left wrist.