I'm doing a re-read now and I'm baffled as to why GRRM chose the ages he did. Sansa is twelve. Arya is ten. Was he hoping to draw in young adult readers, and ended up writing too brutal a series? My daughter is twelve and while I love her dearly, she is not ready to be married off or to kill people.
As it's been said over and over the point is to be realistic towards the type of medieval period the world is set in. During the Middle Ages 12 year old and up was prime period to have children and be married off, especially since life expectancy back then was pretty short and a good amount of your children would die before ever reaching "adult" age. There's a reason young girls are able to have kids and immediately bounce back whereas women in their 30's and 40's have one child and it can take everything out of them. Biologically we're supposed to be having children at such a young age it's just that our societal norms surrounding the subject have changed over time.
GRRM is just staying true to history and as others have said before there was also supposed to be a time jump.