I think it is the format of the teaching and learning that could harm the kids the most- if they are subjected to very structured lessons and tests before they are ready, I can foresee psychological harm done to them, especially in the areas of confidence and dealing with anxiety and stress.

5 year olds are supposed to be in "Kindegarten"- "Children's garden"- a Kinder classroom should have places for the students to explore and learn; most do best in play.

Frankly, almost everyone does best, learning-wise- at play

Personally, I think delaying the start of school until kids are about 7 would likely be a good idea, but it'll never get anywhere because parents would be up in arms about having to figure out what to do with their kids during the day for 2 extra years. ::rolls eyes::