The Infant school that dd1 & ds went to allows parents choose when to start their children (if their birthday falls after January), September or January, and, full-time immediately or part-time for the autumn term. Both dd1 & ds are july babies, both started full-time in September as they had always attended nursery 5 mornings per week, so the school day was only 2 hrs longer than their nursery mornings.
DD1 settled alot easier than ds, but academically she struggled through reception and yr1 (bottom of class in both years), but after the autumn term of yr2 overtook everyone else in literacy, attaining Level 3, in numeracy she attained level 2.
DS on the other hand, took until yr2 to settle in. He struggled academically, and still does. He's at the Junior school now and is much more settled. He was only 4yrs & 6wks when he started the infants, but I don't regret sending him at that age. As others have said, if they start later the other children have already formed their bonds and it may have been harder for him to settle in had I sent him later.
I will have the exact opposite with dd2, as she is a September (2nd) baby and will be the oldest, and will already have had her 5th birthday when she starts school (the school usually has 2 TED days at the beginning of the Autumn term).
Not sure if my waffle has helped coffeebean, but basically you can't tell how things will turn out until it happens.