Its a matter of driving record and confidence. Age only comes into play if someone is very young or very old because of their reflects and reactions.
One needs to be alert and responsible at any age. If they meet the requirements and are comfortable doing long distances, have the common sense to stop when tired and know what to do in case of emergency or accident without panicking, then that is what should be taken into consideration. Only the parent and the child what “age” that is.