16 is old. Not old enough according to our culture but there is no reason we should expect a fictional time-distant culture to have our particular moral or legal prescriptions. It wouldn't be all that surprising if someone from an actual future time to, when reading the work, scoff at how prudish a culture would have to be to consider sexualised portrayals of women that age to be taboo!

Interestingly, if the book in question is the one I think it is, it takes place in Britain, where the age of consent is, in fact, sixteen.