December 31 is the cut off date for school in Ontario. If child turns 4 by Dec 31 they are entitled to start school in Sept as a 3 year old turning 4. What you have to weigh is that they will be in school with the child that is turning 5 on Jan 1 and for some children that doesn't matter and for some they would be playing catch up their entire lives. My daughter is an end of Nov and most years it didn't matter but there were key years when most of her friends turned 12, 13, 16, 18, 21, during the school year previous to her.