For all but two years of my career, I’ve taught mixed-ages of students from aged 2 ½- 6 years old- and I love it!
I find the older children benefit from the responsibility to help care for and co-teach their younger classmates. The younger children achieve more as their older classmates provide an example and incentive to learn.
There’s a huge spread in development and ability and some work is required to differentiate tasks and expectations. I’ve never been too concerned about exposing the younger students to concepts and skills more appropriate for the older students: I’ve always believed in teaching to the top.
Another interesting concept I’ve heard regarding mixed-age groupings is an idea that Early Childhood educators in the 21st century are social-ecologists. Contemporary society has changed from when mixed-aged groups of kids would freely roam the neighborhood and play cooperatively and creatively together. Mixed-age classrooms allow for children to experience a more natural social community.