PamT, for what it's worth my kids' primary school has nothing but combined age classes (except for Reception, intake is 40+ so they are divided into 2). There are then 3xY1/2, 2xY3/4, 1xY4/5 and 2xY5/6. The KS2 classes are of around 35 - the KS1 just about 30. Ideally we would have 3xY3/4 and 3xY5/6 but although the space is there the budget isn't.
My main gripe is the Y4/5 class, the division is done purely by age, albeit on the very valid grounds that parents cannot argue with the calendar. They first instituted this arrangement when the present Y11 (now 16) were in Reception and had a meeting at which they made it quite clear how and why it was to be done (so no arguments about little Johnny having a genius IQ, if he was born in August that's where he goes!)
From my kids' experience, they do perfectly well in a class with younger children, although it can be a pain when they are friendly with the older children in the year but are never in the same class with them - in fact that has been my main problem, plus the fact that if your child is immature as well as young (as both mine affected by this are) then being with even younger children does not give them an example to work up to.
Academically mine have done fine. Maybe they would have done better in a class with older children but we'll never know! It is also frustrating for me because they only spend 1 year in a Y5/6 class and there are two fine teachers but they only ever get 1 of them.
I have always been told that they all do the same work whatever class they're in - certainly they will be prepared for SATs just as strenuously. If you do go into school and ask about it, be warned that many teachers have a tendency to take all comment as criticism (apologies to mumsnetters who are teachers, I'm sure this doesn't apply to any of you...) so be very careful how you phrase your enquiry!
HTH