I agree. You should check with the carrier first. Many times the age of the box can be more important than the age of the unit. We like to keep newer units in the fleet for example. But we do not put a hard fast rule on it. I have seen newer units that are not maintained and are unreliable and junk. I have seen older units that are spotless and well maintained and far better to have in the fleet. I think you should also consider having the carrier pre-inspect the unit before purchase to make sure that are good with it. We require units to be inspected and pass our shops standards. Some carriers will only ask you to provide a valid annual inspection.