Not sure about older children, but I was with a program that ran early childhood for the 6 weeks to 3+ year children (pre-Head Start). Other than the infants, all the children were in the same area. The older kids basically taught the younger children potty training and other skills. The teachers were still able to do individual learning (often helped by the younger children having watched the older children) and the children were happy. I think daycare / ECE that segregates children by years are doing a huge disservice to the children.