See, how hard was that?
Don't worry, Im not here to condemn you and toss you into the pits of hell for being underage. However I would like you to know that backyard airsofting really isnt the greatest or safest thing to do.
For example lets say you and your friends were playing in the back yard and someone walked by who was walking their dog. To the uneducated person an airsoft gun looks like a real firearm. Im sure you can imagine what sorts of things can arise from this, police get called, people freak out, gas prices rise...you know all those nasty things.
Another thing to consider is that what if an airsoft pellet that you fired struck someone else in the eye, someone who was not participating in your event? There are many more examples I could give, however I believe I have provided you with enough examples so that you get the idea.
"Its only a little bit on fire"