my daughter is at a mixing-age group, but in preschool, and so far everything turns fine--but as TimeToTkillTime says, academically speaking, the older ones will be slowed down in advancement. her teachers teach far less compared to those in same-age group of her age. but on the other hand, in terms of social development, at least as the teacher claims, kids in her group know better how to deal with others, older or younger ...
it seems i say nothing useful :-0