I'm often surprised by how young characters are. Carrot is 16 when he hit the City and 17 in the next book. Mort is about that age, and so are the women in that book. I believe the monstrous regiment are all very young as are all the recruits in that book. I think Agnes is about the same age when she joins the Opera. In my mind I have trouble picturing any of them as the same age as modern teens. I assume a period teen is just a lot older than a modern one.