Tigerle, I am curious to know your firsthand experience of Montessori? Our experience across four classrooms at two schools (and I have observed at several others) differs from yours.
Two classrooms at two different schools, one we only shadowed at, one DS9 did a K program at for a year, with a view to enrolling him in first grade there. Each time, we decided against it. Each time, I was so disappointed with how perfect Montessori sounds on paper and how it then was implemented, there was always so much picking and choosing which distorted the whole result. So I started reading up on stuff. My take away (YMMV, must vary really, because you will always see a different implementation) is that you just can't separate the method from the implementation and insist it's still Montessori even though you dont do, for instance, mixed age classrooms. Of course there's always a reason, possibly a compelling one, for why they are doing some stuff and dropping other stuff, but there is also a reason why the method demanded stuff in the first place. So if administrative rules or financial constraints force you to offer age based classrooms, the question is just how much Montessori is left and whether it might not be more honest to call that school "Montessori inspired" or something.
Of course Montessori has curriculum, though it is matrixed and spiral rather than linear. Works are not just randomly placed in classrooms.
Please note that I bracketed "curriculum for 2 yo" with quotation marks. I never said Montessori doesn't have one, it's just not supposed to be age or grade based the way it is in traditional schools, the fluidity and individual pace being of of the points of the mixed age classroom after all.
I don't want to quote your description of the Montessori environment that so far has worked well for your children, just because it would make my post so unwieldy. Suffice it to say it sounds great - Montessori done well, implemented thoughtfully and with some flexibility. However, I do think you are lucky, because it is easy to get Montessori very very wrong and it can work out just as badly for asynchronous kids as a traditional school might.
After all, the OP has asked what to be aware of as her very asynchronous child moves through the age levels, and the age based classrooms are already a red flag that the implementation of those parts of the Montessori method that offer benefits particularly for gifted kids in that particular school leaves a lot to be desired.
