Honostly, its not a huge concern to me as long as they are playing on private property and are wearing proper saftey glasses. ALL of my lil cousins have soft-air guns. They range in ages from 10-16 and frequently have little wars. All of them also wear paintball goggles to play and do so in private forest owned by the family.
But I think back to when I was around 8-12 years old. I played with cap-guns and some VERY realistic guns ALL of the time. We would run around shooting at each other non-stop in public neighborhoods. I am afraid to wonder what would happen to kids if they tried that today.