I have a theory which seems to surprise and offend most people - I think no topic is off-limits to kids, and that "age-appropriate" is a BS term to shield them from various effects of knowledge - like fear, passing on inappropriate information to their friends, etc. But I think not telling kids things - and telling them that they're not allowed to know those things - is breeding grounds for more curiosity about whatever they're being shielded from. It can foster obsession, and they inherit our strange need to hide the information too. In other words, they learn that sex is bad, they seek to find out why, and they hide their seeking from their parents.
I tell my kids pretty openly and matter-of-factly about what's going on in their surroundings, whether they ask about a financial matter, why their dad and I may have argued, etc. Involving them in discussions about things they're afraid of seems to ease their fear and make them feel respected and valued as people who can process information. I think it also encourages a curiosity that will help their intelligence as they grow.
Once again, TMI alert, but one recent example....my 10-year-old son walked in on me in the bathroom when I was menstruating. He said, "So that's what the girls in my class will go through starting about next year, huh?" (I'm not a nudist, but I forget to lock doors.) It's just life.
My point is it's very important to me to teach my kids a sense of level-headedness and desire to learn, and keeping things in the dark impedes that. If they sense that I'll freak out about something, they won't approach me and they'll be freaked out too.
And in my opinion, nothing is sacred. Nothing is too inappropriate to discuss, wonder about, learn about.