"I think you should be consistent. If the age limit is 9, then the age limit is 9. I understand feeling some affinity for the older child, but if you start giving wiggle room on this rule, it will do nothing but create a headache and bad feelings. The older child can volunteer again next summer."
Strict adherence to a rule that seems to have been arbitrarily declared will also create bad feelings. How do you think the kid who's done the job well for years and will just miss the age limit by a little bit will feel about it? Why are the feelings of a misbehaving child far too young for the job, or his mother who lied to get him in and won't supervise him, more important than the well behaved and proven child? What makes you think he'll want to return next summer after being told, "you're great at it but because of the accident of your birth, you're not allowed to do the job"?
There's more than enough grounds to dismiss the younger child without any consideration of age at all, so drawing this particular line in the sand and then treating it like an edict from on high is counterproductive. Letting this kid who likes doing a job that he's proven skilled at continue doing the job is far more important than any consistency derived from following the letter of this rule.