They stress on 'learning at your own pace' and 'learning based on your interest'.
The 'mixed age' concept is quite good in my opinion. It encourages older kids to take care of the younger ones while encourages the young kids to learn from the older ones.
It builds on the child's interests and let the kid develop in his own pace. It makes the kid very self discipline and focused.
At the same time, the social skills could be lacking (as most of the time they focus on their own work and play).
Our son was going through Montessori from the age 1.5 to 2. We didn't continue so the experience might be different as the progress to the upper years.