Yeah, not sure at what point it becomes silly and patronizing and there doesn't really seem to be a period of transition or a specific point/age to say so. They either don't know or they do. Does it become instantly insulting to pretend there's a Santa Claus as soon as they know? Seems like a silly problem, I know. It's important to me personally neither to shit on a child's dreams and childhood, nor to be overtly idiotic about preserving a fiction because of the relationship that we have.