Bitcoin secuirty starts with Pc security, if children knows they should not open the strangers files its pretty safe to use bitcoin. I also think best age is 10 and above.
I think thats true for the most part but anyone can fall for a scam. The more bitcoin is around the safer it will become I believe. The first step to anything is education of it, so 10 + is good. But when would you first teach your kids about money? The same time you do that you should teach them about bitcoin.