I read a lot of posts about reading levels and there does seem to be some anxiety about them. I hope what I'm going to post might be of interest. And might reassure some people who are anxious about children taking longer to get to various levels in reading schemes.
Both my children learned to read with ease. We barely needed the phonics system, they just read the books. Both of them went through the levels speedily. Both scored very highly for literacy throughout primary school.
My son is 13 now. He struggles with English literature, despite being academically bright in most areas and despite the fact that we are a bookish family and there is a lot of reading talk going on here: I am an author and also studied English literature at a highly-regarded university. I always assumed that my children would find writing about texts came naturally, especially as they'd found reading so easy.
Wrong, wrong, wrong.
So don't worry too much if your children don't seem to be keeping up with the bright sparks. Time will tell. Your slower reader aged six could well outpace the fast and bright reader who's shooting through the levels. When he's 13 he might be the pupil his English teacher loves because he writes such insightful and mature essays. Being a good reader is about far, far more than how quickly you progressed through the ORT aged six.