Both my dc started school at the age of 6 here in Germany (dd1 at 6.0, dd2 at 6.3). At first I was a bit hmm but it has worked wonderfully and converted me to the idea totally. Dd1 went to the school's (voluntary) pre-school between 5 and 6, which was then abolished at government level, and dd2 had a free year of kindergarten instead. The main goal of a free nursery year is to ensure that all children can speak German before starting school.
Our experience was that the children learned enormously quickly between 6 and 8, and dd1 at 11 has now certainly caught up with British children in all subjects. And that in two languages.
However, there are certain problems that have to be addressed before 'school at 6' can be introduced. Traditionally, the German system only has 4 years in a primary school before a selection is made for one of three school systems (grammar school, secondary school, and then a school for the children who are written off at the age of 10). Obviously a child who comes from an underprivileged background has very little time to catch up with one who has learned letters and early numbers at home. Secondly, the nurseries have seen their role as teaching social interaction, crafts, music, etc, but have not touched formal learning (because the children are 'too young' for that). Now there is recognition that the system is unfair and is being reformed gradually.
Ironically one of the reforms is bringing forward the age at which children enter school! Now there are school starters as young as 5.6. Unfortunately the teachers (used to first graders of 6.6) are making little allowance for the fact that these children are of course more socially immature.
The main solution would be the introduction of the Scandinavian system - a single school for all until 16. Oops, you had this in the UK and got rid of it because it's so much better tutoring your kids to destruction through the 11+.
The other solution would be free universal nursery between 3 and 6, a (small) component of which is the gradual and playful introduction of letters and numbers. Dd2, now 7 and a fairly average learner, has proved that if children know the alphabet when they enter school at 6, they can learn to read pretty competently within a year.