At a very young age parents instill in children of what sex they are, what are the proper behaviors of each sex and that they could never be the opposite sex. Labeling children at early ages could be at a disadvantage to a child because they will feel some sort of restriction if they were based to do certain aspects. For example girls wear dresses and males wear pants as a quick way of identification. Here in the Us it may not be a norm but in certain countries it is strictly enforced, and if not followed you are being rebellious. Kids get a lot of these ideas from their parents. Many parents have a tendency to "correct" their kids' choices and actions when they don't fit gender norms. Kids pick up on it quickly that straying outside the stereotype is a "bad thing."