Our son is 3 3/4 and we've deferred his start until September 2017.
I used to teach year R and Year 1 and we knew that although academically he'd have been OK emotionally he wouldn't have been.
I agree that all children are different and some will be more ready than others but I also believe that we start education too early in this country anyway (all the research points to starting later being better for young children) and being a summer born child just makes that start even earlier.
With regards to getting a deferral, the education minister did write a letter to all local authorities last year about summer born children saying if parent's thought it was in the best interest of their child to defer then the local authority should agree.
However, the admissions code hasn't been changed yet and not all authorities are following that advice.
We live on the border of two counties and had to apply to both for the deferral. Our main authority accepted our letter requesting deferral right away and confirmed our DS could start in year R next year and stay with his cohort right through his schooling.
The other authority we applied to agreed only after we provided a supporting statement from DS's nursery and when we had their letter through it said they couldn't guarantee he could stay with his cohort right through.
Needless to say we won't be putting any schools from the second authority on our application now.
If you are considering deferring you should contact your local authority to see what stance they are currently taking as it would give you time to think through any evidence you may need.