Marmaduke - it works in Scoltand and no-one seems to have a problem with it.
The "year" is from March to February. That means that technically there should be no-one younger than 4 years 4.5 months whe they start school in August (remember, we don't have Reception).
If you are not yet 5, you can choose to defer until the following year. In practice, this tends to happen with an increasing probablility from November babies onward - but even my ds, whose birthday is 10 Spetember - I would have considered deffering if I'd though he wasn't ready.
For the September to December children, the council will only pay for the 12.5 hours nursery for an extra year if the deferral was supported by the nursery. I understadn that they will automatically pay for Janaury/February children who defer.
So the oldest child in the class could turn 6 just after term starts, but in practice, the oldest probably only turns 6 in October, November, December.
Whereas the youngest could be 4 years 4 years, 4 months and 18 days on day 1. Although there are parents who still seek to get exepmtion and get younger children to start.
So in practice, there is a "drop dead" cut off date - the fact that the child has to start school the year after the school year that he/she turns 5.