I took my daughter dove hunting last year at 3 years old.  I shot a couple times but it was mostly to just hang out for an afternoon.  She loved it.  I don't know when I'll let her or my younger son touch a gun.  I'm going to just gauge it year by year. 

I got into a conversation with a good friend about this.  Our conclusion was that they need to go when they're plenty young so that it's part of their life.  When they are older and you take them out it'll be a hundred things they're getting used to and it might not be as fun for them.  Let it be part of their life even if that means you not being able to enjoy it as much or hunt as well.