Roles that would seem different with age-appropriate actors?

OK. Not sure why it matters. They main point is that they were both much closer to the characters’ ages than most productions.

On the other hand, Arya Stark as a 9-year-old assassin would be freakin’ awesome.

Movies almost always take much longer to produce than you might imagine. There’s no need to go into the technical details here, but 2 or 3 years wouldn’t be unusual.