I know that my nephew and the son of a friend of mine were kept back as both their parents and preschool teachers felt that they were not behaviourally ready to go to school.

This was in contrast to my brother that went to kindergarten at 4.

Interestingly enough both of them are now showing signs of behavioural issues (one ADD, one Aspergers).

Its hard to know whether they will be emotionally able to cope, which is probably why people err on the side of caution.