I believe that it's perfectly fine for kids to have BB guns. However, I think that there should already be some gun training and rules in the home before the child receives one.
For me and my siblings and cousins, playing games with toy guns was part of our childhood. My dad and grandpa were hunters, so we saw guns and knew what they were used for. My dad made sure my brother, sister, and I knew abut gun safety.
When we were little, my dad got some "Eddie Eagle" gun safety cartoon tapes from the NRA and had us watch them. Then he talked with us about guns. We had toy rifles, but we were not allowed to point them at the house, each other, or any living thing. When we carried the toy guns around, we had to make sure the "business end" faced the ground. Even though the toy guns were completely harmless, we were trained to treat them like real guns and would get in serious trouble if we broke the rules. This made handling real guns safer for us when we got older because we already understood the rules.
When my brother was in middle school (probably around 10 years old), he got his first BB gun. We got to shoot it at cans in the backyard. Dad took him to the rifle range to teach him about shooting a real gun. Eventually, he'll probably get a real gun and go hunting with my dad. My DH and I are going to get our concealed weapons permits eventually, and have a handgun to keep in the house for self-defense in case of a robbery.
My brother plays paintball, where the point of the game is to shoot another person. He can easily differentiate between what to do when he has his paintball gun and when he is holding a real gun. I think my dad taught him well.
