When Should I Be Concerned My Child Is Not Hitting Their Milestones? The first thing a pediatrician will tell you is that all children develop at different rates. This is true, to an extent. Human development is marked by what is known as milestones or developmental stages. There is a certain window of growth that takes place at various ages where children will begin doing, or attempting to do, new activities. When a child has mastered a specific task or activity, we say they have reached a specific milestone or developmental stage. When a child has not begun doing activities by certain ages, we say that the child is not meeting their milestones or hitting their developmental stages. One misnomer is that a parent or a pediatrician will say that a child is a 'late bloomer'. Actually, this only applies when the child reaches puberty. It is very crucial for a child, especially from birth to age 5 years old, to meet their milestones and developmental stages within a reas