My DD is 8 and her reading level has been pretty much too high to be measured for about 1.5 years years now. She has always been a strong reader. She can read pretty much anything. She LIKES reading things like the Famous Five and other similar Enid Blyton books.
Obviously with more complicated books though she would struggle more with comprehension rather than reading. She can read all words perfectly, but whether she knows what they mean is another matter.
For example, if I pick up one of my books here, Lydia would be able to read this fine :
"It was just possible, Colin Gomez conceded, that his self-justifications had been just a bit too glib."
But she wouldn't have a clue what it meant lol.
So her reading level his extremely high, but her comprehension level is about that of the Famous Five or perhaps JUST slightly higher.
My DS1 is 6 and he struggles a fair bit. He can manage some Julia Donaldson books, for example the Whale and the Snail, but nothing more complicated that that. Even then he would have to take it very very slowly and sound out each word extremely carefully.
My DS2 is 4 and he can't read yet - I'm going to start teaching him soon though, I'd like him to have the basics down before be starts school in September.