We did this with my DD. We put her in a Montessori preschool and Kindergarten at 3 years old. The first day when I picked her up, I found out that they had moved her to an intermediate class with 4 and 5 year olds that would be entering K the following year. Since Montessori works at the child's pace and is mostly individual tasks, being with an older group did not phase my DD. She enjoyed being with older kids. My DD learned to read at 3 years old in the Montessori preschool setting. The bigger issue is schooling later down the line in elementary and beyond and whether or not they will continue to accelerate your child.
It has been an interesting journey. Good luck!
