My eldest was academically ready but he was also socially ready and had the maturity to sit for decent periods of time and work and things like that. In fact, I often got comments about how the older kids would be off and running and he would sit there finishing his work. He sat at home doing those "pre-k" exercise books for fun. For an hour or so at a time. He really needed to be at school. If I "held him back" til the next year, he would have had a hard time I think. Especially as the friends his age would already be at school.
I think being bored with doing work you already know, can be a huge deficit to an education too. My son was already doing basic maths (2+5 etc) and was able to write and was at the early stages of reading. He would have continued to learn during that extra year at home, but when starting school he would have had to sit through lessons of what he already knew. So he essentially would be wasting a whole stack of time, not learning anything new.
I only have my own experience with this to go off and basically long story short, moving interstate and the new state having rules about what grade I should be in according to age, meant I was put in a grade lower than I should have been. My mother says that this was a very big down turn in my work. Again because I was bored and not being challenged anymore. So I sat twiddling my thumbs and not giving a crap.