Chattel slavery as we practiced it in the US was certainly wrong. However, "slavery" as it was practiced in Ancient Rome was not at all like the slavery that we practiced here. In fact, it more resembled modern employment than chattel slavery. No, the slaves could not just leave when they wanted, but they had rights that had to be scrupulously cared for, including the possession of their wages. They also had guilds to which they could belong that invested their earnings to help them purchase their freedom.Secretary, librarian, teacher, chef, accountant, physician, dancer, artist, government bureaucrat - just some of the occupations that were entirely servile in nature. Only soldiers and lawyers had to be free men. (By the by, it was the soldiers who built Rome's roads, not slaves.)
As for the ages of consent, when I said that it was moved about like the rope at a bank, I meant nowadays, not the past. All over the World, it varies.
As for the past, w/a much shorter life span, getting married at age 12 was not unreasonable. As life-span has increased, the span of childhood has telescoped to an almost ridiculous length. And we have invented something that did not exist in the past: adolescence. To a large extent, this has to do w/delaying the entry of even more young men and women into the work force. The lack of experience that goes w/being young was, in the past, treated for what it was - lack of experience, not a special disease that afflicts the young.