Hi, my DS is end of August and has just started Y2.
In reception he was absolutely fine/ahead academically. Emotionally IMO pretty mature with the exception of some anxiety regarding rules. But he struggled hugely socially.
I think on average, just-4-year-olds are at a very different place regarding being able to deal with peers. And going from a pre-school setting where peer interactions are usually monitored by adults who intervene when necessary, to a school setting where the kids are pretty much left to themselves unless they actively seek out that adult intervention, is a huge step.
DS was treated badly by one boy in particular. He was spat in the face, pushed, manipulated (e.g. boy promised to buy him treats if he'd give up his place in the line, or that he'd buy him Lego if he stopped being friends with another child), excluded, made fun of, called names.
DS had had no previous experience of any such behaviour, from anyone - and was totally confused. Just didn't get it. Kept trying to do things for that boy and wanted that boy to be his friend.
The problem is that a young child will learn that being treated like that is ok (rather than outrageous) if nobody intervenes, and once it has happened once or twice with no consequences, will never think to tell a teacher about it. Because the child will not understand that this is the kind of behaviour that one should tell a teacher about.
My DS also learned that 'telling' is bad. Friends don't 'tell' on each other and he so badly wanted to be friends.
For these two lessons that he learned, I regret not having deferred his start. However in a different school, different group of peers, he might have been fine, so I don't think this is a general rule.
How did we deal with it? Once we became aware of the extent of things (he never told us anything, because he didn't realise how awful things were - he just became more and more subdued) we talked to the teacher, who kept an eye on things, and steered DS towards other children. This worked for a while but after a month or so the teacher said the two boys had played beautifully and the topic was closed for her from then on. However that boy continued to be mean outside of sight from the teacher. In fact still does.
So we had lots of chats with DS, working out strategies. Such as 'walk away', 'find someone else to play' (best to have specific ideas of WHO he could play with instead), and 'tell a teacher'. We also worked out what he could say in response to mean things.
Until DS was solidly 5, so well into Y1, he was still trying to be friends with that boy and was still being constantly hurt and rejected and belittled. It was only after he had reached a degree of 'social maturity' that he realised that it was not his fault that the other boy was being mean to him, and only as he gradually built himself a small but strong group of friends, that he eventually was able to say 'X is often mean to me and therefore I don't want to invite him to a playdate/my birthday/...'
Of course bullies come at all ages and there is nothing to say that DS wouldn't have been bullied just as much if he had joined the class later. But I do believe that he might have had just that much more time to develop 'social maturity' and might have been able to see the other boy's behaviour as what it was - mean and manipulative.