Speaking from the perspective of a Mom with nine kids, six of those boys....You can never start talking too early. And, you should never tell them more than they want to know. They're very good at letting you know if you pay attention. With both the boys and the girls, just start talking...casual conversation is better than long detailed lectures. A general conversation about body changes....it's a rare 10 year old that hasn't realized that teenagers and adult women have certain attributes that little girls lack, for instance. Perfect time to talk about modesty. Just looke around and there's plenty of conversation starters.
Nine kids... awesome! Thank you for the advice. I've already had talks with all of them about modesty and appropriate dress and conduct. It's the next phase that has me a bit nervous :-) Thanks again.