How do you determine what level of language to use for children and YAs?

Here’s a comprehensive follow-up to yesterday’s post about readability scales. This article, from a library site, was sent to me in response to a comment I made on the Web of Language blog. The article explores many of the scales available. I’m curious: how do writers of young adult and children’s books make these decisions? Is there a scale or set of guidelines widely in use for this purpose? If so, how does it compare to the scales and tools discussed here? I’d love to hear from authors of YA and children’s books. What tools for selecting the right level of language would you recommend?