DS is the oldest in his year. I was given the choice of an August or a September baby. We thought September was a no brainer but they said apparently lots of people chose August because it's a whole additional extra year of childcare to fund. (Funding has changed since DS was born).
DS was friends with a lot of the older children who went to school the year before him. This was hard as he had to start over when they left.
When they left it absolutely wasn't ready for school. But he definitely was by Christmas that year. The difference was just a few months difference in practice. I felt that year was definitely an 'extra' over the youngest kids.
He is by far one of the most mature. It really shows at times. He is much more on a part emotionally with the youngest of the year above. But he significantly has a lot more confidence than the youngest in the year above. Compared with some of the youngest in his year the difference is huge too. There's a massive range of when they are ready for High School too. Which also isn't necessarily linear with birth at this point but you've already made that decision.
There's also frustrations. Being at the beginning/end of a school year has multiple ramifications. For sports he's having to wait an additional year as the cut off remains 31st August. He can't train with the older kids so he's stuck at a training level he is past. For Cubs and Scouts he ends up out of year because he's gone up with the youngest of the school year above - it means he hasn't been with his year group peers. At times this has meant he's struggled with confidence in that situation as he doesn't know the others and they have pre-existing relationships from school within their year. Then by the time the youngest has come up, DS has done a full cycle of activities and doesn't necessarily want to fit back in with the younger ones.
When we get older he will be the first to be able to get driving lessons and drink. This puts different pressures on kids and it puts its own associated responsibilities on the eldest which the youngest don't feel. As a young driver with his friends that's a scary prospect that he will be the one having to make the grown up decisions whilst his younger friends do the dicking about.
In deferring you make a decision to separate from peers. It puts more pressure on them in future years. More is expected of them if they are the eldest which can be a good thing for some but for others it's really not as it adds a layer of responsibility that some just aren't really ready for.
DH and I are both summer babies and noticed it even at university. We wanted things to be the other way round to us and I think this weighed heavily on us - so it really wasn't a decision we made based on DS himself.
If I'm completely honest though now I'd say neither youngest nor oldest is 'better' as such. Being either inevitably brings its own challenges regardless - if you are the eldest / youngest in the year it's unavoidable and you do get 'what IDs that those in the middle don't consider. It's a case of picking your poison more that 'the better' option.