I listened to a child psychologist who specialized in sexuality (I wish I could remember her name)talk about sexuality and children. She said that if a child is asking questions, you can tell them age appropriate answers to their questions as long as you stick to science, nature and health/safety...it's logical... no matter what age. Also, only answer the question they have asked....if they want to know more, they will ask more. Too many parents get overwhelmed and feel embarrassed and believe that they have to spill the whole encyclopedia on sexual behavior once that "can of worms" has been opened. I've also read that if they're at the age where they are responding with "Ewww gross!" ...you've waited too long.
When I was a kid I didn't know what self-gratification was, as a word, but believe me, I was familiar with the feeling of it way before age 10. I think it's safe to assume that most children do. 10 years old is likely too late. Children are curious way earlier in life. If you leave it until they have asked questions of their peers, you can bet they'll be getting some pretty strange answers and perceptions that could be damaging to the present and future health of ther sexuality. We are naturally sexual beings. The more relaxed and understanding you can be of the questions your child will be asking of you, the more likely they will think and believe that the connection between their bodies and sexuality are natural and healthy.