For what its worth, I would have to agree. The paranormal (unless pushed on them by inheriting a gift) is no place for children. Its not to be taken as a game, and even the most matured teens don't need the type of potential baggage that comes along with it.
Its no game, and should never be treated as such not by young or old alike. Unfortunately I've seen quite a few adults who treat it just as bad. But the difference being when one is older at least its a choice. Where as when younger the full consequence may not always become apparent until its too late.
As with anything of course its a personal choice that parents have to make, but my personal opinion would be to wait until they're older. And again, as you mentioned, the exception would be in the case of them having special gifts. In those cases, its no longer a choice of if but rather how to best approach it.