Examination of the ages at which children achieve motor milestones shows that:

I believe that the examination of the ages at which children achieve motor milestones show that there are wide variations in the ages at which infants begin to sit, walk, and crawl. Developmental milestones are the skills such as taking the first step, smiling for the first time, and waving which vary differently from one child to another. Children reach milestones in how they play, learn, speak, act, and move. For example, between the ages of 9 to 12 months, children begin to achieve physical milestones such as standing up or even walking. 

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