The range we go to states in the rules that 16 and younger must be supervised by an adult withing arms reach at all times when at the firing line or otherwise handling firearms. They are relaxed about this with my sons and me but we still comply to the letter of the rule when at the range on busy days so as to not create a problem with young shooters who are not as well disciplined at range safety as my boys are. My boys are 16 and 14 years old now and have been shooting at the range since they were 7 years old.
I would clarify exactly what the range rules are regarding age. If that jerk was not going by the rules he must be reprimanded by the Rangemaster. If there is a minimum age rule then you should have been informed of that fact when you arrived with your sons that day.
If there is no minimum age rule I would go to the Rangemaster and ask that I be given a refund for the day that I was asked to leave, or a free day at the range.