I started my kids and grandkids on shotguns when they were large/strong enough to handle a 20ga. For the most part that was about 12. I do not share Toby's enthusiasm for 12ga guns on beginners. At skeet or sporting clays where you select the targets for training purposes the 20 is indistinguishable from the 12ga as far as success goes. I also hand loaded light 3/4oz loads for them. Select open chokes like skeet or IC. The biggest detriment to a young/small shooter is ill fitting stocks. If your 9 year old is of normal size, you might want to consider something you can purchase replacement buttstocks for cheaply as you are going to need to cut it way down and grow it back about every year. And a beginner certainly doesn't need a double or repeater. I hunted on a single shot until in college and don't recall ever being short on game- usually the opposite as I knew one shot had to be carefully used. Many of my hunter education students do well on the remington junior model 870 with magazine plugged completely so gun is single shot. My grandson was started on a Charles Daly (Miroku) 20ga but he was a large kid and could handle the gun. Watch out for cross dominance in the eyes. Trying to shoot shotgun right handed and left eyed will generate a very poor start for youngsters. Just switch the gun to the dominate eye side they will pick it up just as well.
