Back to the somewhat serious topic at hand:
It hasn't been that long since the average lifespan of humans was closer to 40 than 80, and when you die "young" the ability to reproduce at a
young age is a (species) survival trait. In my opinion, it's not so much a new thing as it is an old thing that seems unusual to many of us because
we've "evolved" past it.
Many early societies and "primitive" tribes have rituals that pass children into adulthood at age 12, or thereabouts. Religious confirmation
is an adaptation of a "rite of passage into adulthood" ceremony, and doesn't it happen around age 12 or so?
Across the species on planet Earth, there is a clear statistical correlation between the age of maturity and how much there is for the offspring to
learn. Horses, for example, although not fully developed, are basically "adults" at around one year of age. I mean, really, all they need to learn
is how to eat grass and run fast.
The more intelligent the animal and the more "sophisticated" the social structure, the longer maturity takes. This is basically true of humans as
well, in my opinion. As our world has become increasingly complex, it takes longer for a child to learn everything it needs to know to survive on its
own successfully, thus we have high school until age 18 and college which takes one into the early 20's. In more 'primitive' societies with much
lower levels of technology, there is much less to learn, and thus children are considered adult at a much younger age. A human can learn to be a
successful hunter-gatherer, or a successful farmer, in 12 or 14 years, but learning to write computer programs or be a doctor takes a bit longer.
What interests - and puzzles - me is how this necessity has been translated into an ethical/moral issue. We view young adults as "immature" at 16 or
17 because they haven't yet learned everything they need to know to make their own way in the world (or support a family), but how has it become a
issue? Nature (or God, depending on your beliefs) determines the age of sexual maturity by the onset of puberty, and this occurs much
earlier than the late teens. So, how have we come to the belief that sex with a physically mature (as defined by puberty) human is morally and
ethically wrong, just because they aren't intellectually and educationally mature?