I don't think it depends so much on the age but rather on the kid. I have an 8 year old boy and 10 year old daughter and I take them regularly to the range to shoot real guns. I teach them about safety and they understand that it is not a toy.
Come Christmas time, my son wanted an air-soft gun. I didn't hesitate to buy him one once we went over my requirements (always wear glasses, don't aim at people, etc) much like I did with the real guns. Since it was his first air-soft gun, I made sure it was of low power just in case (after all he is only 8). Up to now, everything has worked out perfectly with him and with my daughter.
Therefore, it will really depend on the trust you have with your kid. The issue I see is that you cannot only base the decision on having an adult ALWAYS near by as this not necessarily be the case. Also, I see a lot of the older kids in the neighborhood actually having battles with their guns. That type of play is potentially much more dangerous than just shooting at targets.