I think that Visser Three's whole character though is sort of this power-hungry egomaniac to contrast with Visser One's more subtle approach. It's not like the story doesn't have enough depth to acknowledge that Visser Three is the apex of Animorphs villainy... you know? It's a little more nunanced than a cartoon villain, or to give a specific example, better than how Jim Carrey played Count Olaf in the Lemony Snicket movie. V3 is recognized as having relatively shallow intelligence amongst the Yeerks, he just retains his power because he's incredibly violent and ambitious. Or at least, um, that's how I remember it.
And I don't think they're too old, really. They're 3-4 years too old, not 6-7. And just IMO, there's no way a studio is going to greenlight a blockbuster movie with 13-year-old kids going through the incredibly violent types of atrocities described even early on in the books (severed limbs, etc.) I would think in the movies they would have to be a bit older: say, the war taking place as they age from 16-19 years old (as opposed to 13-16).