The longer you wait the better, imagine telling her at 13, when all she will be thinking about is clothes, boys and how to dress etc etc. You would sit there and explain to her how you go airsofting and problably get into big details while she would be thinking "Whatever dad, your just a geek" or something.
Telling any young child about this sport would either invoke confusion or unwanted attention. Its best just to wait until you gauge that she is mature enough to understand that its just a game.
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