DS is a late August baby and started reception at only just 4yo. Academically he was more than ready, if anything he wasn't challenged enough. Socially was a different matter though, it was towards the end of the school year (he was still over a month away from turning 5) when we felt he was about 'ready' for starting the social wilderness that was school (breaks, playground, lunch time).
He is smart and perfectly 'school shaped' (square peg in a square hole!) so he sailed through his primary years with little effort, great results, rarely noticed, academically bored, using his entire energy and interest on navigating social stuff/friendships.
I did at times wonder, perhaps if we had delayed his start, he would have been more comfortable socially (that first year was tough!), plus, then he would have been not just 'top of class' but properly standing out, which perhaps would have gotten him into some 'gifted' programme or something and he would have had some challenge, instead of just being left to coast as he was in his 'proper' year.
During Y6 we moved abroad and here his birthday falls just on the other side of the cut-off, so he is in theory one of the oldest in the year below rather than youngest in current year.
There is more flexibility here and perhaps if we had pushed, he could have gone into the year matching number of years at school, so into the 7th year of school rather than cohort, which is the 6th year of school, so essentially Y5. We didn't push though, as there was a new language to get to grips with, plus many, boys especially, start a year later, so he would have been youngest by nearly two years or something in the year above. In his now new 'correct' cohort he ought to be one of the oldest but actually isn't, as so many of his classmates started late. He did have an extra year of school though (doing Y5/6th year of school twice). Plus here the first two years they do not learn any reading/writing/numbers at all, so in fact DS has had three years more academic learning than his new classmates, despite being in the correct cohort.
So it is not really surprising that despite the language challenge, he was identified as 'gifted' within a few weeks of starting, and was enrolled into the school's gifted programme. Which meant he got to skip one morning/week of regular classes and spend that time doing projects with a small group of other 'gifted' kids instead. This was exactly what I had wondered when musing what might have happened if we had delayed his reception entry back in England.
We have been here for three years now and have to say from this experience:
Academically, being youngest in year and among the top of the class, and coasting along with little challenge, was much better than being 'oldest' (not really) in class, being streets ahead of everyone, being recognized/labelled/separated out as 'gifted', being bored to death 80% of the time, and getting that 'gifted' workshop one morning/week for extra challenge.
Socially, a later start would have been much better. Starting at just four was too early for DS, but even if starting later but within cohort, being youngest among his friends brings its own challenges. I really liked how after our move, he was allowed to be a child/his own age much more/longer. In contrast to always trying to keep up with the kids who were all older than him, taller, more mature, into more 'grown-up' things, he was able to be running around playing catch for longer.
To me, with DS, the social side outweighs the academic. The lack of academic challenge/growth/stretching, is something we can address at home, but if he is socially miserable at school, there is little we can do to help. I'd rather he spend his days bored at school but happy with his friends.
It depends on the child though and some kids will truly suffer from lack of intellectual stimulation for such a large part of their waking lives. And get nothing from the relationships with similar aged kids. So they might weigh the factors differently.
So that's my view/experience as parent of a bright, academically ready summer born. In our case, delaying would probably have resulted in an overall happier experience.