I don't really trust SST "ages" to be perfectly honest. Grover doesn't act like Elmo and Zoe and Abby. Ernie and Bert are, for all appearances, grown-up enough to live on their own. You don't see their parents dressing them or anything. I can believe that the characters TARGET certain age groups, even if they're not those ages themselves. Using simple language (for the most part) and being rather innocent doesn't make one a child. Only age makes one a child. Of course, there's always the concept of "mental age", but I personally don't like thinking that nearly every Muppet character on the show is maturationally delayed to such an unbelievable extent.