I think that statement has a lot of "if"s behind it. Sometimes children are wiser than adults. Sometimes children go by a simple rule "If it's not good for me or someone else, don't do it." Sometimes children don't have the right answer at all.
By the time you are grown up, you should have a definite feeling of what is right and what is not right. Society expects common sense from adults. They should have enough worldly experience to be able to act correctly.
