flip, thanks for responding, even if only tentatively for now.
I realize the question can be disturbing to many people. Perhaps a little more information may be helpful in adding new dimensions to thinking about this topic. I've been exploring primitivist/anti-civ critiques of the way we live currently, as well as reading what I can of differing indigenous peoples.
One of the counter-critiques of anti-civ I keep seeing is that it's inherently ableist and perhaps even homophobic on some deep level. This led me to ask the following:
Anok's answer contains within it the direction I was going when they ask the deeper question between civ and gender. I think this is applicable here as well to some extent. As I hinted in my initial comment, I think people confuse understanding with condoning, particularly in our (post-?)Christian civ. I think a hasty conclusion of a slippery slope then ensues: 'well, if 18 isn't a good age, why not 8 in your book?' This kind of thinking. It's prevalent.
However, I've come across no instances where a tightly-knit band society diddles with kiddies on any kind of scale nearing what happens in, say, the US. Even while these peoples have sex and mate earlier, there doesn't seem to me any of the weird-ass fetishizing of young people like there is in moralistic, repressed, alienated, 'civilized,' societies such as ours (I'm speaking more of the US. Europeans with greater contact with extended families may not be as prone). It seems to me that most, if not all, of these band/tribal peoples live more intimately with one another on an everyday level, for one thing. There's far more touching and 'sex' itself isn't so compartmentalized like we have it. I just don't think the desire really arises for large disparities in age on any large scale. I would suggest, too, that any tabus which arise in these cultures may arise after such an event and perceiving the destruction it leaves, rather than it being a moral rule prior to its occurrence.
What are the relations between sex and civ? Between our bodies and civ? Between touching others and civ? etc?
But, like Montaigne, I have to voice my uncertainty: Que sais-je?
Edit for typos/additional thoughts.