For my own children, they were ready when they were 4 years old. They weighed over 40 pounds and both of them were tall, so that their shoulders were higher than the slot for the shoulder straps for the 5 point harness. So it's not so much about age as it is for the physical readiness of the children. If they can stay in the 5 point harness then do so, but for many 4 year olds that is not possible.