Re: How young to start?
I think you already know the answer. With your boys already being introduced to fishing at a young age, there is no reason hunting cannot be the same. Make it fun, and call it a day when things get boring. Bring a shotgun for grouse, and the necessities to cook one. Kids love grouse, and are always hungry. Take them on hikes that they can handle, but don't drag them around the bush. Drive a little, hike a little. Bring a fishing rod for the afternoons when things are slow and the animals are laying low. Most importantly, teach them the ethics of a respected hunter, along with the conservation of wildlife.
The measure of a man is not how much power he has, it's how he wields it.