Apparently, the youngest children in any school year face more stress than their older peers and are at greater risk of developing mental illnes. A survey of more than 10,000 children in Britain between the ages of five and 15 showed that regardless of when their school year began, if they were the youngest in the class they faced greater stress.
It is not a huge stress like family problems, trauma or being in an accident, but it is a moderate stress and, given that it applies to a lot of kids, it is a serious health threat.
British Medical Journal August 30, 2003;327(7413):472 Free Full Text Journal Article