Airsoft does hurt but it's a sliding scale, in 20 years i have seen many teeth shot out ( under 18 must wear a full face mask at most if not all sites) i have seen BBs stuck in peoples skin, i have personal got a few scars from being shot and have come home covered in welts, i have shots that sting all day or a few seconds. surprisingly the pain of being shot in the genitals goes away very quickly. I have taken shots that i have heard more than felt but in the end the greatest risk of injury is loosing your footing, breaks, dislocations, torn ligaments, sprains.
but 90% of the time it just stings for a bit. however airsoft is most definitely not for 8 year olds and even though some sites let 12 year olds play i would say unless they have done some serious growing minimum age for airsoft should be 14.