Just to add to what Emily has said, I would check with your local Council. I know that in our area, it is the September after their 4th birthday (my dd was almost 5 when she started), but in some areas of Wales, they have a dual intake, with the next round of children starting in the January after they were 4 (so my dd would only have been 4 and 3 months).
Try telling that to my 14 year old who says 'Mum you have no idea how hard it is at school.' I know its a long time since I was there but why do your children never believe you attended school.