I did not defer my summer-born twins, (born August, and 3weeks premature) though I felt in my heart they were just not ready for school. They were close in age to my oldest, and were slower in development than my oldest.
Whern the time came for their assessment at school, I was reassured they would be fine, but I had severe doubts. I wish I had had the courage of my convictions because they struggled for several years, not just academically but socially and physically, in sport, etc.
Circumstances in year 4 led to us moving school and it was suggested they repeat Yr 4. They never looked back from that moment. They went on to acheive very well and have both graduated from Oxbridge with excellent degrees.
What I am saying, I think, is that you probably have a reasonable idea of your own child and their stage of development. I don't think that holding back a child who might just cope will lead to any real detriment - they can always be given extra work, further reading, or extension work if it becomes clear it is needed, and 'accelerated' if necessary, but not holding back a child who really needs it can be a disaster.