Bigkidsdidit... The data about no long term benefit to deferring, or indeed to deferring actually harming kids is trundled out in one form or another occasionally by councils trying to discourage parents from exercising their legal right not to start their child at school until the August after they turn 5.
The data, when you look into it typically looks at the attainment of children who have (mostly) been deferred for medical and social reasons which have seriously impacted their development - not for more general reasons of parental choice or for being not quite ready for school.
Sadly, many of these children, even tho they may benefit from deferral, will still do worse than average with their schooling... But I bet the extra year in nursery helped them cope with school better than they would have without that deferral.
For kids who are progressing well, who are maybe just a bit young socially IMO the benefit of not enrolling them at school until you actually have to, is the extra year they have to mature, being a little bit older when they come to cope with exam pressures, uni applications etc.
If we had the time again, our eldest two who started p1 at 4 1/2 would both be deferred (despite both being academically very high achieving). Both were sitting Highers at 16, uni interviews at 16 1/2 (against English and international students many of whom were already 18), started uni at 17 1/2 after completing 6th year...
There are a lots of valid reasons for not rushing to start formal education at the earliest possible time, it's not just about wether they are "ready" for p1 at 4 1/2.