Here are a few reasons:
- older children modeling advanced work (and yes, advanced obnoxiousness sometimes...) Even if younger children aren't ready to do the harder work, they see it and look forward to doing it.
- greater ability to group children according to ability (younger children working with older if needed, and vice-versa) provides for more advanced work, and/or remediation as needed.
- just a small new group of young children each year, rest of class hits the ground running at the start of the year so classroom is running smoother, sooner.
- spending three years in the same classroom with the same teacher builds a strong community between children, teachers, and parents.
- children experience the progression of materials from very first introductory lessons to higher-level work, progressing through new materials or progressing through concepts with same materials.
- older children are proud to reach that point of being the leaders in the classroom, and often enjoy and benefit from helping the younger children.
- if they stick with Montessori, they start all over again in 1st grade being the youngest in the class, once again exposed to higher level work and that anticipation of becoming the leaders.
There are more reasons I'm not coming up with off the top of my head, but hopefully others can come up with more to reassure you that the good far outweighs the bad! But as I always say...I'm biased from teaching Montessori for so long - wouldn't know what do do with just one age group!