If you charge different rates for different ages then for sure the child would stay at each rate based on development rather than age - when I did infant/toddler /preschool rates the time to change rates was when walking and feeding self table foods and only using cup at daycare move to toddler rate then stayed at that rate till completely potty trained to reach the preschool rate. So a child that is 4 but not potty trained and needs to be fed by me would be on the infant rate still.

If you don't have different rates then you could see about a rate differential if the parents are willing. Remember if they are supplying foods or keeping child home on therapy days but still paying you that you are in a sense being reimbursed extra money the other families aren't paying.