I don't think this is a real spoiler but it is one of the issues of the show from the books. In the books all the children are younger. Robb and Jon are both 14 I believe and Dany is also 14. Thus Sansa is 12 or 13. Basically the time from Robert's Rebellion to present was shifted from 13 years to 17 years since at the end of Robert's Rebellion you had a "baby boom" of sorts. They had to change that to 17/18 for the show because of the sex, you couldn't exactly have Dany with many of her scenes with Drogo if she is a 13 year old actress and it simply wouldn't play well with audiences to see that insinuated either. The reason Martin wrote them as that age is it is more appropriate to the Middle Ages where you didn't really have adolescence, you were either a child or an adult with nothing in between. For women they became adults as soon as they had they "flowered".
It does cause a few inconsistencies but really it is the only practical way of doing it.
It does cause a few inconsistencies but really it is the only practical way of doing it.
