I started bringing kids along at 3 - 5, depending on the kid.
Just make it fun for the kid, and keep in mind his/her ability to concentrate, sit still, etc.
Make it a great hunt, not just about getting a turkey.
One thing I learned in taking my 5 kids hunting when they were small: Just make it fun. Animals have a way of showing up when you're just having fun with your kids. And even if they don't, you will be making amazing memories.
You'll also have the best chance of growing some amazing hunting partners if you take them out early, and keep it fun.