I've just found the piece I read about Bo's age in The Dukes of Hazzard: The Unofficial Companion. Here's the bit after John liked the script and wanted the part of Bo.
"My agent told me, 'You're too young for it, and you're from New York, so we told them you were twenty-four years old and from here,'" Schneider recalled. "They were looking for people from the ages of twenty-four to thirty. I was eighteen, just graduated from high school. If they knew that, I would never have got an audition."
I've also had a quick flick through Redneck Boy in the Promised Land, but Ben just describes the part he auditioned for as "the good ol' boy mechanic with a wild and crazy streak". There's no mention of age.