Shucks. I've carried a knife since I was 3 or 4. Don't really remember exactly.
Shooting, I started personally around 9, I guess. But I grew up watching my older brother shoot his Single Six off the back porch. If I wanted to know, I just had to ask.
I'm a long way from kids, but I think they'll let you know when they're ready. I'd say answer questions when they're asked, and be honest. Don't downplay the danger, but do emphasize the positives. If they want to handle them, ensure they're in a safe condition, and let them handle them, emphasizing the rules.
Worked on me, anyway. Been shooting for a good while now, and received my first "own" gun for my 18th bday, another at Christmas, and I've bought myself a few since then.