Each child is unique. It is therefore difficult to describe exactly what should be expected at each stage of a child's development. While certain behaviors and physical milestones tend to occur at certain ages, a wide spectrum of growth and behavior for each age is normal.
These guidelines are offered as a way of showing a general progression through the developmental stages rather than as fixed requirements for normal development at specific ages. It is perfectly natural for a child to attain some milestones earlier and other milestones later than the general trend.