I don't know - here in Ontario, Canada, children currently start school at the age of 6, however, there is half-time Pre-Kindergarten for 4 year olds, and half-time Kindergarten for 5 year olds.
Having said that, my daughter is home schooled, and could count in 6 different languages before she was 5, has travelled to around 9 different countries, reads books from the adult section of our local library, has volunteered for a local organization for 10 years (meals on wheels, with me) and she is only 12 years old.
Putting her in school at 4, 5, 6, 7, or 8 as far as I am concerned, would be such a waste of time, she is getting an excellent education - she has been a member of the Girl Guides for 8 years, sings in 2 Choirs, plays piano, plays soccer in the summer, is an active Church member.
One of my concerns about bringing the age of first school attendance is this - a hundred years ago, my grandmother went to school for probably 7 years, from the age of 6 until she was almost 13 at which time she started working full-time. My grandmother was well read, intelligent, knowledgeable about the world around her, hard working and was up to date on current events around the world,
There are people today who started school at 4 who can't find Botswana on the globe, who are incapable of writing a coherent letter, who can't spell (without spell-check), who have poor General Knowledge, who can't perform basic Mental Arithmetic ..... need I go on?
Despite the start of schooling getting earlier and earlier, standards are dropping. My suggestion would be to start children at school at 6 years old, but turn the darn TV off, unplug the video games, take away the X-boxes and DSIs, and have children learning about the world around them by taking them to Museums, Art Galleries, going to see how things are made, visiting local farms, and going to parks and beaches - there is so much for children to learn outside of school, but these days millions of children are cooped up in schools for the majority of their waking hours, for at least 14 years of their lives - what a shame for the poor kids.
Me, I teach my daughter at home, with the help of home school groups, where, in the past three years, she has learnt Cooking, Archery, Chess, Knitting, Logic, Woodworking, Ballet, Highland Dancing and I wouldn't trade what we do each day for the freedom of sending my child to school at 4 years old,