Hi,
It depends on the school, the classroom, and you and your daughter, really. How's that for a helpful answer?
The reason that most Montessori schools want to have children start at three is because it really is a three-year program, and though it is true that children are given freedom to work through it at their own pace, there is a logical order to lessons that are presented. Each lesson is linked to the next, and when the child starts doing "academic" work (writing, reading, math), she is building on skills that she gained earlier. It may be difficult for a child who is completely new to the environment to pick up on classroom culture, as well - there is a lot of freedom, and the older children are expected to be able to work independently for the most part.
However, many M. classes will accept a few older children who they think will be able to make this transition and fit in well with the already established class. It is frustrating to find out that so many won't, and it may be partly where M. gets a reputation for being elitist. But in actuality, it's mainly because many M. guides feel it's really hard on the older child.
It's always worth a try though - I'd talk to some administrators and teachers and see what their thoughts on it are. Good luck, I hope you find a good fit for your daughter!