Put the Appalachian Himalaya and Rocky Mountains in order from youngest to oldest knowing that the Himalaya are most rugged and the Appalachians are the least rugged?

Rocky Mountains are the youngest, followed by the Appalachian Mountains, and then the Himalaya. The ruggedness decreases in this order, with the Rocky Mountains being the most rugged, followed by the Himalaya, and then the Appalachian Mountains as the least rugged.