As far as legality goes.... most states allow minors to drink at home under the supervision of their parents. Ethically..... I personally see nothing wrong with a child being educated in the entire process of brewing. In a similar way that a majority of kids that are raised around guns and taught how to properly handle and respect them, children brought up around alcohol will also realize it's not something to be abused.
My 6 year old helps me quite regularly when I brew. He helps with the grinding, hop additions, and yeast pitching. Just can't figure out why he always seems to disappear when the cleaning starts.
Brewing can also be quite an educational experience for any aged child. Be it the actual science of starch conversions, measurements, or as simple as instilling green principles (compost, reuse of water, hop gardening, etc.). Just don't shove it down their throats, you'll be able to bring up brew buddies of your own.
I've already told my boys that when they turn 18, we're going to make a batch of mead and a batch of barleywine for them to open on their 21st birthday.