There is mounting evidence in favour of delaying formal teaching of abstract skills (reading, writing and arithmetic) until pupils are developmentally ready.

In order to learn to write, a number of prerequisites are necessary.  Among others the following are required; concentration, visual acuity, motor dexterity and grasp of language.

What does the evidence tell us about the development of these prerequisites?

Children’s visual acuity reaches adult levels between 4 and 7.  Motor dexterity, in particular fine motor control needed to manipulate a pencil, is not usually fully developed until at least 5 years and studies suggest it is slower to develop in boys.  In the first years of life children are rapidly learning spoken language.  As any person who has spent time with young children knows the rate of language acquisition is very different between children.  Studies show that between the ages of 2 years at the earliest and 4 years at the latest children start to grasp complex grammar (negations and questions) but they don’t often perfect it until later and are often still gasping language subtleties until the age of 10.

In summary this shows that development is very different between children.  One child may be ready to learn to read at 4 but another might not be ready until 7.  It may very well be the case that most (over half) of children are ready for formal tuition aged 5 but it is arguably also the case that nearly all (90%+) are ready by aged 7.

What is the impact of this 2 year delay?

This is practically impossible to test experimentally.  However, our European neighbours tend not to progress formal teaching of abstract skills until the age of 7 and often overtake English students in reading writing and maths test by the age of 8.  This suggests that being developmentally ready allows the children to progress much faster.  It also makes sure that far fewer children develop aversions to reading, writing or arithmetic from having been forced to engage in a task that is physically or cognitively beyond them.

http://news.bbc.co.uk/1/hi/education/7234578.stm

What would children do during these 2 years?

Delaying formal teaching until the age of 7 does not mean children cannot continue to go to school, nor does it mean that children should not be developed.  Lots of activities can be done with 5-7 year olds to prepare them for reading, writing and arithmetic in away that is physically and psychologically gentle on the child.