I agree.
Children know far more than most adults think. Unless you've been raised in a very insular community, as Our Lady said, "There will be almost no innocent souls, no, not even among children." I take this to mean not that all children will have succuмbed to sins against the 6th and 9th, but that their minds have already been exposed without their conscious awareness. Whatever enters the eyes or ears is there for life, all the easier for the world, the flesh, and the devil to fan into flames later on. Think! If you take your year old child into a supermarket or a drug store, he rides in the little seat, and sees pictures on magazines of immodestly clad women and men. It doesn't need to be nude or outright porn. No, he (or she) has no interest or desire at the time, and will "forget" it, but later on, those pictures, even if not consciously recalled, can be used by the enemy when puberty arrives.
So is the solution to live off grid in total isolation or to join a bizarre cult? No, of course not. It is to maintain an open, honest, caring relationship in which children feel, even when they've done wrong or are in their adolescent years, that they can come to you at all times, with all questions, in any circuмstance, and be heard, given wise counsel, and that even punishments are backed by your love.
Children who cannot do this WILL fall into mortal sin, if not impurity, then something else, leading to impurity. Children so unfortunate to be raised in groups like certain Amish, Hasidic Jєωιѕн, the polygamist Mormons, on ashrams, upon leaving and "discovering" sɛҳuąƖity as accepted by the majority, nearly all cast off every restraint. Satan loves ignorance.
The solution is also not to deliberately expose them to perversion and then tell them it's evil! Satan loves miseducation, as well. (This is why school based sex-ed, anti-drug, anti-alcohol, anti-bullying programs don't work; no, not even if done in traditional Catholic schools.) I heard an SSPX school, not sure where, had some sort of class for high schoolers.
Dad, you did the right thing! Teach according to each child's understanding, temperament, and as circuмstances require. Teach by you and your wife's Godly example. Do this and when your son encounters the wrong thing, his conscience will already be formed to reject it. If he's tempted, he'll know how to pray because you've already taught him. He needn't know why he prays the three Hail Mary's or calls upon St. Joseph, model of purity, or girls, to St. Agnes. But when the need arises, the graces given at Baptism, by regular confession and Holy Communion, by Confirmation just before the onset of puberty, if at all possible, will enliven his heart and mind by the Holy Ghost. He will will not merely avoid the sin, (remember, willfully indulging in mental impurity is mortally sinful, not just the act), but achieve mastery over his lower nature.
Add my comments to the "Things I Wish My Parents Told Me" list and sign me "Penitent."