The ear reaches its full growth when a child is about five years old. Operating on a body part before it has reached its full growth can stunt or deform the remainder of that process. Teasing begins at about age five or six, which is about the earliest age otoplasties are done.
At this age, boys and girls can follow instructions and cooperate, to an expected degree. Yet they are young enough to typically not object to or resist surgery. But by age eight it is no longer purely the choice of the parents. Beginning around that age, a child is old enough to voice that they do not want to have surgery and certainly can have the strength and will to say no to the procedure.
By the early teens, children on their own may start to ask for ear surgery, often when the parents never noticed that there was a problem. While some parents will notice prominent ears on their children, the beauty of their own child may be so great that they never notice, or it may be too subtle for the parents to think it matters. But if there is an issue, the earlier the surgery is done the better the outcome because the ear cartilage stiffens as we age, making it increasingly difficult to create as ideal of a shape.