In one of the earlier episodes(Can't remember which) but it shows bender being born, and full size on an assembly line, then gets his "bending" degree(Which also raises further questions, because he said that Robot house was his frat house during bending college... But on the line its "uploaded" into him). He also says hes was made 4 years ago
Rewatching Teenage Mutant Leela's Hurdles, I noticed bender gets littler and littler...
Farnsworth explains it. The chroniton-saturated tar has attached itself to the robot equivalent of DNA, meaning that Bender will reverse-age to match the humans. Because it would be less funny if he didn't.