My class C was an (older) teenager when I bought it used. I didn't finance the purchase, but paid cash. I think it's safe to say I follow the antithesis of this "rule."
I'd venture to guess that this rule, if it actually exists, was formulated by dealers or finance companies as a way to keep their gravy train rolling along. Rolling over the financing into a new vehicle doesn't seem to me to be a winning idea in the long run, but rather a technique for getting deeper and deeper into debt, assuming I'm understanding what is going on properly.
