The excellent answers so far are very Eurocentric -- it would be interesting to hear from scholars specializing in other parts of the world too. From my observations while traveling, this seems to vary quite a bit by culture and it is not a universal taboo even today. I don't want to get into specific anecdotes because that's not what this subreddit is about, but, for example, it'd be illuminating to hear how thinking about this has evolved over time in African cultures.

On a somewhat related tangent, when did the idea that children became adults or old enough for sexual identity / activity at 18 appear?

Can someone tell me how children and sexuality/privacy was viewed amongst slave communities?