I think the age of Arya, Bran, and Sansa are the least of their concerns.
I think:
- GRRM expected to be done or at least way more done than he is now
- Show outpaced his books.
- The bullets GRRM gave are probably hard to actually write into reality... the show has struggled putting it together in a believable way. I'm sure its even more difficult in the book(s).
https://www.theringer.com/game-of-thrones/2017/8/24/16196178/george-r-r-martin-won-season-7-a-song-of-ice-and-fire-books