Earlier quoted context omitted.
It's meant to mean 18 year old adults, or woman around that age (so it could be applied to 17 years old as well). This has nothing to do with child pornograpgy, but it seems to be a trend of our time to use that word when youth pornography is meant (and judge the latter accordingly).
A 17 year old would not be legal, so I don't see how barely legal applies to 17 year olds because that would be illegal in most jurisdictions. EDIT: Thanks, I misunderstood.
My point was, it has nothing to do with children.