I'm going to come at this at a different angle entirely. I have an older & a younger sister, my older sister was born in January, so went to school just after she turned five. She was never very academic. My younger sister was born in June and went to school at four and a half (hers was the last year where the cutoff age was turning 5 by the end of June). She was (and still is) a social butterfly & very gifted academically. I think from that point of view it is purely a personality thing.......

The biggest issue was that my younger sister was in an "older" grade so to speak, a lot of her friends turned 18 in Year 12, where she was 16 until the middle of the year. She had problems from a social point of view that she couldn't go out with her friends because she wasn't 18 yet. From an academic point of view, she finished uni & was a fully qualified teacher at 21.