The difference is that departments should be taking full advantage of the loophole in the law that allows those under 21 to drive in non-emergency situations.
Do I think the rule requiring age of 21 to drive in emergency mode is silly? Yes. What makes a 21 year old magically a better driver than a 20 and 11 months year old? I think a properly trained licensed driver should be allowed to drive. Are there 17-20 year olds not capable of driving? Yup. Are there 21+ year olds not capable of driving? Yup.