Interesting that you have challenged me, rather than the person who is equally confident that it was the best thing for their dc to be deferred.
Also I didn't say "certain" I said "fairly confident" which isn't the same thing.
There have been children deferred, but not in these numbers, and often they were children with significant needs in the past. I think it's interesting, but research needs to go ahead to show if there is an overall benefit, because if there isn't then they need to be looking at other ways to support those who need support.
I am "fairly confident" because he's a bright lad, who does just enough to get through to the next level. I can say with experience of him, that he does not benefit from being ahead, instead he gets frustrated, bored and then goes off into a world of his own which is rarely helpful.
He also has made some lovely friends who were in his form and they have stuck together and in the 6th form are still sticking together. He's had opportunities that he wouldn't have had if he'd been in another year (eg due to GCSE timings), one of which is potentially leading (having made contacts) into the career he's decided he wanted to do.
He'd generally had teachers that get him, not entirely, but pretty much so, eg he had a year when he was badly ill and missed most of a term and his teacher (to my surprise, having had my older one and not been that impressed then) was fantastically supportive and really helped him through it.
And well, he might decide he wants to marry one of their siblings, but currently he's planning on living alone with 4 dogs, 4 cats, a goldfish pond, 10 budgies, one cockatoo, a grasssnake and a hamster. He says no wife will live with that. :D