"Bad-mouthing" anyone serves no purpose for anyone, parents or otherwise. Peace, but also hatred and anger, begin at home; and I don't care how old anyone's kids are; I think it's the role of parents not just to be an example, but also to increasingly present reasoning and logic and understanding to family members (and if not "present" it as kids grow up and gradually must process more and more information/facts about the world (their own, their family's, and the world-in-general), then to at least promote those things).
It's one thing to say a child (again, regardless of how old he is) that you don't agree with someone (and then say why, and if it makes sense to your child he can sort out his own ideas about the matter. It's also one thing to state to a child (especially young ones) what is right and wrong, why you believe it is (and sometimes it may be wrong just because a child is too young, while other times may it may be just wrong in general or else may not be entirely black-and-white). It's even sometimes a legitimate thing to tell a child (and I'm going to make up an example here) that you hate one individual and that the reason you hate that person is that he did "x" that harmed someone, or you. Made up example: "I don't like so-and-so because I saw him abusing his child/pet." Another example: "The reason I won't go in that store is because I hate the guy who owns it. I went shopping in there once and he made a really inappropriate move."
Ideally, we want our kids to learn how to sort all this stuff out (through example and with reason and with a certain amount of objectivity when objectivity is required). We want them to learn that experts and authority are not always right for one reason or another, but we want them to respect everyone (ideally). I think it's even yet another thing to name-call another driver who almost causes you to be in an accident. That person may be being an "idiot" at the time. Kids understand anger at such a thing.
Wild, out-of-control, "bad-mouthing" most often comes from lack of understanding or something else that we really shouldn't be modeling for our kids (again, no matter how old/young they are); because once kids are more out in the world and their own life they already "relate" to/see the larger world/life on their own terms; so encouraging peace/understanding then if often more about within the family. Breeding aggression (even verbal) and lack of understanding/respect is destructive.