“Yes and no – the ‘taboo’ as we currently understand it (ie, the expectation that children should be more or less sexually innocent) dates from the eighteenth century.
Prior to that, children tended to live in close quarters with adults, and they spent much of their time living and working with adults in settings where it would have been pretty hard to keep them from learning about and observing adult sexuality. But that doesn’t necessarily mean that children were allowed to see everything or to act on all of their own sexual impulses. According to the doctrine of original sin, children had an inherent tendency to do wrong and corrupt things, so adults often punished or admonished them for sexual curiosity or sexual behavior that was not in keeping with how good, christian adults were supposed to behave. They may not have been seen as sexually innocent, but they were still expected to live by the same rules that everyone else observed – which means that their sexuality was often tightly controlled (depending, obviously, on what specific time and place we’re talking about).
The late seventeenth and early eighteenth century brought a massive change in how adults looked at children; as European economies and daily life became more complex during this period, parents who were members of the emerging “middle class” began to realize that their children would need a prolonged period of skills training and education if they were to be successful in life. At the same time, philosophers like John Locke and Jean Jacques Rousseau started to argue that children were “blank slates” (tabula rasa) by nature – that they were not “filthy bundles of original sin,” but rather more or less innocent at birth. The effect of these changes was that more and more adults started to worry about how their children’s adult character and chance of success in life might be affected by what happened to them and what kind of environment they lived in as children – so they began to show more and more interest in keeping children innocent, and keeping them more or less ignorant of sex. The fear of masturbation was particularly pronounced here – there’s a famous work from the early 1700s called Onania: the Heinous Sin of Self-Pollution, and All its Frightful Consequences, in Both Sexes Considered , which warned that masturbation caused a whole range of different physical, spiritual, and psychological afflictions. The book was a raging best seller that went through tons of different editions during the eighteenth century.
Another example is Rousseau’s book Emile: On Education , and was published in 1762. It was supposed to be about educating the perfect citizen, but it placed a great deal of emphasis on the need to keep children from masturbating lest they lose their inherent goodness and capacity for rational thought.
So while adults did exert a fair amount of control over children’s sexuality prior to 1600 or so, it was only after that point that the notion developed that children were innocent by nature, and needed to be kept that way. The enlightenment’s view of children as “blank slates,” the growing importance of education for the elite and middle class, and the protestant reformation all played a key role in brining that transition about. It was more or less complete by the early 19th Century, which is when we first start to see the development of romanticized and sentimentalized views of children – and especially girls – as an embodiment of innocence itself.
Keep in mind, though, that all of this really only applied to middle- and upper-class children in Western Europe and the US. Slave children, working class children, immigrant children, etc were not really “covered” under this idea that kids needed to be protected from adult sexuality – and many of them were exposed to sex or even sexually exploited from an early age well into the twentieth century.”