I don't know if
@DragonParadox is using it as a reference, but there was an official, GRRM endorsed d20 game game for Game of Thrones back in 2005. I'm not sure how far into the book setting its material is, but there are PC levels assigned that should be pretty useful to determine what named characters could reasonably reach.
Gregor Clegan, for example, is Man-At-Arms 6 / Knight 4. Drogo is Raider 10 / Commander 3. Jaime Lannister is Man-At-Arms 1 / Noble 3 / Knight 3 / Brother of the Kingsguard 7.
Given that we are several years pre-canon, actually closer to a decade, all of these I mentioned, plus most of the others in the book, should be at least a few levels lower.