We as a society have decided that the time you spend inside your mother does not count as your age, and that's why we celebrate Birth Days to mark the passing of each year.
We could get more medical or scientific and say that while you're inside the uterus, you're still developing as a human being and you're not formed 100%, so we decide that when you get out, it's because you're fully formed and ready to rock the world.
In Korea if i recall correctly they are 1 year older because they account for this time, but again, it's a decision as a society.