The dates are different as I am in Scotland but I was able to keep my dd1 back until she was 5 and a half and will be doing the same with ds. In Scotland the school admissions year is done Jan-Dec, with children born Nov - End Feb able to go into school at either 4 or 5, and as it is the calender year no child starts school before 4 and a half. They then stay in the same year, (with the people they started with) right through school.
So my ds, born Feb 2002, (4 now) will start school in Aug 07, but he COULD start in August this year. But he's having another year in preschool instead, adn I am confident that I have made the right decision.
My dd1, born Feb 2000, could have started school age 4 1/2 in Aug 2004, but we kept her until 2005. It was so the right decision for us, and she was an outgoing, friendly, confident 4 and a half year old. In our area, most of the other families with 'winter babies' as it is in Scotland tend to keep them until they are older, which led to a very lively nursery class for her, with plenty people her own age around and she thrived on it. Scotland doesnt have 'reception', children go straight into Primary one, though there are still 7 years in Primary (but only 6 in Secondary), so the extra nursery year was a bit like reception for her. SHe has done amazing things in her first year of school and has loved it, and I think thrived as she is that little bit older and so able to cope with it better.
Also (puting my secondary school teacher hat on) I see abig difference in children coming into secondary at 11 and at 12. The 12 year olds are so much more mature adn able to cope with the change. This is especially obvious with boys, but holds true for girls as well (maybe one day I will do some research into it, as every teacher I have worked with comments on it but I have neer read anything about it).
I know most of this is irrelevant to you as you are obviously NOT in Scotland, but it does seem wrong that you shoudl be forced to put your child into school when they just do not seem ready,adn that there is so little flexibility about dates. I hope very much that you can work things out to suit your DD2