If the entire world around you insists that Santa Claus is real, you are not dumb for believing it. If you grow to be a teen and then a young adult and the whole world keeps proclaiming Santa Claus is real and you never saw your parents putting gifts under the tree, you are still not dumb for believing it.
I don't think belief is mostly intelligence-driven. It is highly emotional and includes much from our evolution, especially the desire to explain things that we haven't understood, so we say "God did it." Our evolution also gives us the desire to belong to the group and the fear to stand out against what the group says and does.
Once a person breaks from the emotional part, intelligence can come in. A person who stops believing in Santa Claus because he gets old enough to figure out that poor children get worse presents than rich children can also figure out that the miracles of his belief system stories are just as ridiculous as the miracles from the belief system stories that he scoffs at.