I personally think Montessori is beneficial to all ages. But, kindergarten marks the end of the 3 year cycle, so it's when most people who will transfer in the first place actually do it. A lot of people justify the cost for preschool and kindy, but it's hard to justify the cost for the older grades I guess. But my DS is in 1st grade at a Montessori and is already working on things that many children in public schools don't touch until 3rd grade or higher.
If you plan to send your child to public K, and she's already missed the 3 year old year, you'll be keeping her in for just 1 year, right? While I think that she'll get some benefit, she'll be spending time learning the work from the 1st year (although it won't take more than a couple months), and she won't get to see the conclusion of the 3 year cycle. Most of the works are a progression that starts from the first year and goes through the 3rd years. So, if she is in for one year, she'll get the benefits of that year, which I think are great, but she'll miss a lot of the big picture. For example, the pink tower isn't just a stack of blocks. There is a progression through 3 years with that pink tower that starts with learning to care for the material and experiencing the process of stacking, and learning the basics of mass & size and eventually that same work leads to learning about volume, how many 1 cm cubes could fit inside the largest (10 cm) cube, which eventually also leads to cubing, multiplication by 10's, etc.
