This was some time ago so don't know if it would be that helpful, but I missed out what was then "third year juniors" (5) and went straight from "second year" (4) to "fourth year" (6).
It was not an experience I particularly noticed, I have to say, even though I was in a class with older children. I tended to play with the children from my original year at playtime rather than my classmates so didn't bond that much with the new class, didn't have any best friends in it etc. One downside was PE - I was a May baby so one of the younger in my year anyway, now I was doing sports with children who were sometimes nearly two years older, it did make me feel I wasn't any good at sports, something that stayed with me for a very long time.
The other downside was, as others have said, having to repeat the year - in one memorable case the same actual test/exam - but in this case it was my own decision, as the school wanted me to take the 11plus and go to grammar school a year early and I didn't want to. (The grammar school did take some chidlren a year early).
There were two other children in the same boat and when repeating the year, apart from the same exam thing, we were generally given stuff to do by ourselves - French workbooks, more advanced maths, reading comprehensions etc. Possibly the curriculum was less set back then as a lot of the classroom work was not the same both years, projects on Belgium (!), maps, the Victorians etc.
Don't knwo if that is at all helpful.