yes... Id assume that the field owner had the parent/guardian sign whatever waiver was needed so that he is not held responsible, much like they get our adults to sign. 18+ printed on the box is not law but recommended age, and if a parent/guardian feels their child isnt responsible enough to play with one then they shouldn't allow it and take it away, thereby giving the parent/guardian final say and responsibility with their childs actions.
If any player gets shot by a minor and can sue the field owner, then if a player gets shot by an adult he can be sued just as likely, and this has nothing to do with age of the shooter but the fact that the field owner did not legally cover his ass.
Friendly Fire...... ISN'T!!