Children develop mostly in the same sequence as that is the order than their bodies are meant to develop. Say, a baby can't start walking at 2 months when it can't even sit up by itself. The body has to develop in ways where the rest of the body will support this. They will develop at different rates as the hormone levels for every child will never be the same at different ages. This is for many different reasons, such as stress, smoking, drinking etc. All of these can slow down the development.