Aaaah... First Grade / Class One readiness is a pretty big topic. That said, it seems that we are discussing cut-off dates here, not readiness so much, which does say something. A lot of school seem to do the same thing.
There are several cut-off dates that I have heard of.
This site, for example, states that the child 'should have experienced seven Easters on Earth' before entering Class One. So their cut off date seems to fall somewhere in April. (This means the child should have turned six by that time.) Other schools use the end of May, or even of June as a cut-off date.
I have heard of schools who wait until the children are already seven, too, but that seems pretty wrong to me. I'm not sure I can explain why --partly just because it feels so-- but I will try. For a start seven seems pretty late to
begin to learn to read (and knit and count and all of that). Secondly, if you wait until 7 to start Class 1, it seems to me that the child is always a little ahead of the curriculum. As children approach 8, they move out of that dreamy, my-teacher-is-the-best-thing-ever mood of Class 1 and into the trickster energy that is best met by fables. You don't want that to happen while they are in Class 1. As they approach 9, they begin to 'wake up' in a whole new way and they are capable of so much more independent work -- this is Class 3 territory, you don't want it happening in Class 2.
And so on and so forth. Of course all this is somewhat subjective, and also relative, as different children do these things at different times. They are, however, also influenced by the state the whole class is in --in fact that class often becomes a being in itself-- and it seems to me that the more older children you have, the faster it is going to move ahead.
But to go back to
readiness, I think children generally make the move from kindergarten consciousness to Class One consciousness somewhere between their 6th and 7th birthdays, and so I would
consider any child that had turned 6 by September first. That said, it is unlikely that a lot of summer children would be ready, and it is likely that even children born in the spring would not be.
Sometimes I think a lot of this confusion comes from the fact that Steiner, in my translation of the Kingdom of Childhood at least, talks about the
seventh year, and people forgetting that the seventh year is the one before the 7th birthday, not the one after it. But, not being able to read German, and never having bothered to check with the original, I may well be wrong on that.