Grammar school or not, I can't believe it makes that much difference, but I always felt cheated at school personally having been born at the end of May - what I potentially could have achieved given a extra nine months!
There is another way of looking at this. My two DS's both now at GS are summer born, one end June and one end July. They both excelled at primary and doing great at GS too. My view is that being what they are, being one of the youngest in the class has given them the competition that they need, something to kick against so to speak, would that have been the case if they had been born a couple of months later? - especially while at primary. So maybe it all evens out in the end. Anyhow, it's likely that if they're good enough to get in, they're up to it, irrespective of age. But then I'm November born and needed all the help I could get, so what do I know!