In answer to OP's question:
As a teacher in KS1, I have always found that MOST summer borns are usually 'lagging behind' the autumn borns. Not all of them, there are some who are 'exception to the rule' like most things. But as time goes on, that usually changes.
By KS2 (which I have taught in), most summer borns catch up, not all, but most. In which case, you can probably ascertain that those who haven't are probably 'true SEN'.
Every year when I get a 'fresh batch' of Y1s, those on the SEN register are all May, June, July and August born. This year there has been one exception. One child is Feb born and another is March (but had no reception teaching). I tend to 'take them off' the register as I don't believe they are 'true SEN'. They are just delayed due to age -not due to lack of ability.
Take a baby born in September 2011 and another who will be born August 2012 (bless, not yet conceived!). When the August one is born, the September child will be almost one. A big difference in development. It will probably be no different come September 2016 when BOTH will be starting school - one turning five while the other may have only turned four days before.
To cut a long explanation short, not all summer born children will be 'disadvantaged'. Most children MAY find it tougher, but there are exceptions to the rule.