July-born DS was always the smallest in his class, but not quite the youngest, and we discussed him staying back but were advised to hold it in reserve so that the "spare" year could be used elsewhere, for a scholarship attempt perhaps. (At 9, his struggle to write turned out to be poor visual processing skills, and any possibility of a scholarship evaporated.) He has just gone into Y12, having largely caught up his cohort in size, emotional maturity and academic results in the last 18 months. However, if his A level results were predicted to fall very short, we might consider a third year of study in pursuit of MC advantage!
Purely personal and anecdotal evidence comes from DNs. One was born late August and the other early September, and their educational outcomes were polar opposites. One was always smallest and slowest; the other among the oldest and most advanced, and the consequences of not having the option to defer starting school has followed them into adulthood.