Complete ignorance with regard to certain matters is perhaps the best thing for children; but let them learn very early what it is impossible to conceal from them permanently. Either their curiosity must never be aroused, or it must be satisfied before the age when it becomes a source of danger. Your conduct towards your pupil in this respect depends greatly on his individual circumstances, the society in which he moves, the position in which he may find himself, etc. Nothing must be left to chance; and if you are not sure of keeping him in ignorance of the difference between the sexes till he is sixteen, take care you teach him before he is ten.

Emile, or On Education (1762) - Book IV