i learned about it when i was 4-5 years old too from watching animals and this wasn't so far back as the 1920s or the 50s.
it is different to watch animals and learn from/through them than to learn about what teachers have to say in school about human sexuality.
to learn such a thing through the lives of animals, gives you that sense of feeling that it's just part of their nature, just like eating, pooping out in the open etc. for example, no parent would have their young child watch porn. well, by watching animals do it, i never considered it as such and it was a natural thing for them, they did it in the barn, out in the field, everywhere. you can't prevent your child from seeing that.
however, it is part of our nature too. but if you tell a child such things, then they are going to get very curious. everyone knows that if you tell a child not to do something he is going to do it. in a study regarding this that i saw on TV, they placed a child in a room with cameras which had toys, they told the child there are toys behind him and that he is not allowed to turn his head and see them nor play with them. eventually all kids gave into it.
now through such education in school, kids will want to try it out, they already are from as young as 11 or 12 due to pornographic images that they see on the internet but with such education they will feel that they can 'play it safe' because they know the dangers.
Funny theories about kids aside, I have a friend who raised two daughters to be ignorant about sex. He simply told them to abstain from sex because he figured that telling them about contraception and so forth would encourage them to have sex. Naturally, both daughters got pregnant out of wedlock before they were even 20 years old. Ignorance is more likely to get kids in trouble than facts.