It depends on the kid. I have three kids - two boys. The oldest is going with me and actually pulling the trigger for the first time this year. He's 10 and has hunted anything he could around the yard with the pellet gun. He probably could have gone at a much younger age. My 8 year old can't sit still to save his life. I will take him at age 10 also, but I don't have high hopes.
So, it depends on the individual, but it sounds like your nephew is a good candidate.