Personally, I wouldn't say that just because Johnny is 4 he needs to go to preschool. The preschools here are either in a center, head start, or an outrageously expensive two hour/two day a week program (seriously, about $100/month less than average full time daycare rates).
Of course, I have very definite educational philosophies, so that may be part of it. But if a parent felt that Johnny would benefit from going to preschool, I certainly would not try to talk him/her out of it. I may ask if there is something I could do to assist if there is an issue of K readiness, though.
Children who are ready to learn will learn. Age is just a number.

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