technically you could be 10 as long as you have an adult (over 18) supervizing so there is no need to be 21.
Only to buy it, other than that shoot away
I suppose it is up to the parent. Most airsoft fields don't allow kids under 8 to play. That is the best answer I can give.
18 To by them 11 Up to fire them,
To use one legally would depend on local legislation