If it is established by competent evidence that an injured employee is under eighteen years of age and his incapacity is permanent, his average monthly earning capacity shall be deemed, within the limits fixed by sections 23-1041 and 23-1046, to be the monthly amount which under ordinary circumstances he would probably be able to earn at the age of eighteen years in the occupation in which he was employed at the time of injury, or in any occupation to which he would reasonably have been promoted if he had not been injured. If the probable earnings at the age of eighteen years cannot be reasonably determined, his average earnings shall be based upon four dollars per day for a six-day week.