Why are these cats still suckling/kneading people?

My brother adopted two kittens who were taken from their mother at a very young age. They are now about two, and they still suckle and knead people whom they are cuddling. I understand that the method is for nursing and increasing milk flow with the mother, and I understand that they started doing it with people because they were taken from their mother too soon. What I'm asking is, why do they continue to do this as adult cats? Are they always going to try to satisfy an unfulfilled kittenhood need? Is there anything wrong with letting them continue to do this?