I have to agree with Aemilius Paulus. Especially in the more "civilized" countries like Rome, the average diet wasn't exactly delightful, it consisted mostly of bread or anything made of cereals. We know from the upper classes of Egyptian society, that they had extremely bad teeth due to the sand which enters the flour upon the process of grinding. And that was the upper class.

Of course we cannot tell with certainty, but I don't think that girls reached puberty at such a young age as today. Even if upperclass girls weren't malnourished, I doubt that it would be at the age of 12. I think that the house of the parents had a close watch and wouldn't allow inappropiate behaviour.

And last but not least, having a mistress is nothing to be looked down at, and a much better imagination for me than a twelve year old giving birth.