Brain - at our LEA the summer-born-ish kids start in January when they are at least 4.5 years old. I always thought that this was standard but apparently not.
Round about 3 years old I started to do literacy and numeracy with my DC. Nothing Einstein-ish. I would simply get him to count the number of peas on his plate for example or the number of coins in my purse. As for reading, basic flash cards with pictures and like 'car' and 'cat'. In addition to this he went to play groups where there were older kids.
Basically, by the time he started Year R he was used to being around older kids and academically he was equal to his older peers.
No, this is not me being smug. I am merely making the point that the problems mentioned upthread are not inevitable.
As for starting school later I don't think that this solves anything. Various reports have talked about how some kids enter Reception with poor numeracy and literacy. With some kids its because the parents believe that young kids should be free of academic stuff until they start school. With others it's because the parents are unable or unwilling to support their children.
So, if the starting age were to move to 6-7 my DC will have spent those extra years doing numeracy and literacy with me. Consequently he will start school 1-2 years ahead academically speaking, compared to a child with a less pushy involved parent. Being behind by 6-11 months is quite insignificant compared to the gap now being opened up by pushy parents.
IMO non-PhD opinion any effects from being summer-born should have dissipated by the time that the child leaves primary school. If it hasn't then the parent needs to seriously look at what they themselves are doing or not doing with their DC. The teacher and TA can only do so much when they have 30 kids to teach. There are already provisions for SEN or kids starting school with no English. Most schools don't have the resources for another set of provisions.