I don't think i would put an age limit on it. As a parent, you should know what your child can and can't handle and also what you want them to handle.
I have seen younger children do fine so far at Halloween Haunt and I also have seen a few not make it past International Street before hiding in The Emporium and have to have Park Police come and take them to the front because they were too scared to come out of the lighted store (true story).
If the haunts are a little too extreme for the kids, think of it this way.... You don't have to go into the Haunts if you don't want to. You can ride the rides all night. Yes there may be some monsters running around the areas, but they can be avoidable.