RE: what age to start
I just did a guided turkey hunt with a 7 and 9 year old. Great kids, but they clearlyweren't ready for hunting yet.
First off, I don't think they fully appreciated the firepower in their hands. Secondly, I don't think they had really formed the desire to be there. It was more of a show for dad than anything else. They would have rather been home asleep or playing Playstation.
Physically, they had trouble keeping pace, couldn't carry their gear, and if they would have gotten a turkey/deer, they would have been unable to get it out of the woods on their own. No sense of direction either.
In my opinion, a kid should start hunting at that age where he/she has formulated the desire and exhibited the ability to hunt. That is, they should be big enough to carry their own gear, responsible enough to safely handle their own weapon without an adult having to watch over their shoulder. They should be able to responsibly handle a bad situation (lost in the woods, or a broken bone, etc.). From my experience, that doesn't really happen until age 12-14.
Just because they can use a scope and can pull a trigger doesn't mean that they're ready. You don't give kids drivers' licenses just because they can turn a steering wheel.