my daughter is five and is keenly interested. She has been able to tell the difference between a horse fly and a honey bee for some time :)
so far I let her look at empty frames and boxes when I'm working in them and I explain what's what, and she's had busy workers flying all around her, but I think it will be some time before I let her come with me to open a hive.
She does understand that bees will sting and that hives are off limits. I'm trying to acquire a couple of very very calm and quiet Queens, so she may learn as she grows up if she should happen to maintain the interest.
I think the child needs to be old enough to not panic, to be able to understand that they need to control themselves if they get a visor full. I know adults that can't :)
At school there were teaching hives, grade six and up - that's 10? 11? years old?