Sticking strictly to book descriptions probably gives a good idea of who is more likely to be cast, but I mean, come on; isn't Asha described as smoking hot but the show "Yara" is anything but that. Xaro is white, show Xaro isn't. Show children probably are aged to also give the world more appeal. (along with the violence/nudity) I could easily see Jojen/Meera being older so that Bran's story makes a little more sense on the TV screen, a group of small children trekking through the dangerous North? Aging up children involved in mature storylines is pretty common, and I don't have a problem with aging up Meera at all.
I don't care if she's hot or not, that will probably not affect the casting at all. They'll pick the best Meera regardless of looks because her looks, frankly, aren't all that important for the series (as opposed to someone like Dany, who has a distinctive look and is a primary character).