I don't know about "proper", but a smart kid will start picking it up pretty early - probably as soon as they begin to have experience of other adults outside the family - neighbors, teachers and so on. Probably some awareness of the differences will be starting by about five or six years old.Most would know by the time they are eight.
I see no problem with that. Most teens see their parents as that, in a few cases they are quite right. It is what they do with their knowledge - is it not.
I left home at 17 because of this, never looked back