If the injured employee is under 18 years of age, and his or her incapacity is permanent, his or her average weekly earnings shall be deemed, within the limits fixed in Section 4453, to be the weekly sum that under ordinary circumstances he or she would probably be able to earn at the age of 18 years, in the occupation in which he or she was employed at the time of the injury or in any occupation to which he or she would reasonably have been promoted if he or she had not been injured. If the probable earnings at the age of 18 years cannot reasonably be determined, his or her average weekly earnings shall be taken at the maximum limit established in Section 4453.