First year in a mixed-age class went very well. It was a class for 3 and 4 year olds (oldest were 5 years, youngest turned 3 during the school year), our son was 3,5 at school start. Excellent teacher-assistant combo, nice kids, only one "problem" kid (a good boy, but pretty wild), involved parents. A success, and we felt our son really got a lot from being with slightly older kids.
Second year, new class. Not good at all. Now the Troll was the absolute oldest in the class (first to turn 5), over half of the 4-5 year olds were boys and out of them there were now 4 problem boys (one of them violent, and one of them totally out of control). Add to the mix a big group of 2-3 year olds, of which twins who didn't speak the language and weren't 100% potty trained and one very cute but totally hyper boy. Parents ranged from uninvolved to unable to contain their kids, and the teaching staff definitely didn't cope well with the group. The Troll went in total Rain Main mode and withdrew from the rest of the group, didn't care what was happening and threw tantrums every morning when leaving for school. Only good thing is that he was always very nice to the younger kids, unlike some of the other boys his age.
Last year, big switch to a small Montessori School with one class for kids 3-12 years old. 24 kids, 4-6 adults (teachers and assistants) present and extremely involved parents. It's paradise. The older kids help the younger ones, but at the same time they have their very own projects that they work on individually or in small groups. The younger ones work according to their abilities and fields of interest. The Troll is actually looking forward to school (most days at least).
So, based on that, I would keep an open mind, but I would look carefully for signs that anything isn't as it should be, and I wouldn't wait an entire year before reacting if there was a problem. We waited way to long and as as result it took a very long time to build back our son's trust and interest in school. But really, that conclusion would also be valid for a normal, mixed-age class.