How very scientific. Have you written any papers or done any studies on the subject? I'd love to see what you've published.
At the beginning I decided not to accept any students under age three in our school. Their distress (in the form of ear-splitting shreiks) was so obvious and so painful for me, a mother too, that I did my little part by setting an age limit. We also tried to steer parents in the direction of in-home babysitters rather than large daycare centers which have continually rotating personnel.