I know there will be differences, some children seem too young even by the end of Year R, others including a couple of young relatives I have are chomping at the bit to get started.
Scotland start at P1, so most skip starting at the age of reception, but then I have just seen this www.gov.scot/Publications/2012/05/7940/4 which is similar to the American system for starting Kindergarten (which is our year 1) and it can be deferred, which for some reason they call red shirting in the USA but it seems some parents in Scotland can also defer entry, I know technically you can defer in England but you do seem to be expected to jump through many many hoops..
Most of the continent start later for formal schooling-and are still quite play based when they start.
We tend to start at 4 (with some lucky children very nearly five) with full days quite quickly.
Sorry it is a rambled post