My son was 8 weeks prem (and 1lb 15oz at birth), and therefore born in late July instead of mid-September (what terrible timing!). When the time came to start school he was absolutely not ready for reception (he wasn't even reliably dry in the day!), so we kept him at nursery, and then at home as I was on maternity leave with #2.
He skipped reception altogether. I taught him to read, count etc at home, and he started school in year 1. He was (and still is) very immature for his age, but with each year change he gets used to the behavious levels required within a term or so. He was tiny (at aged 5 he was the size of an average 2 year old), and this caused him some problems initially as the other children thought he was a baby! (He is now average height for his age, so still smaller than most of his classmates, but no longer at all bothered by it).
He had fewer problems adjusting to year 1 than most of the other children who had been in reception; they had been seeing school as this great fun place with sand and water play etc and year 1 was a rude awakening. He, however, associated playing with his time at nursery, and was happy with the new environment and took it in his stride.
Academically he was way ahead of his peers in reading, and quickly caught up with maths.
It was definitely the best decision for him (and incidentally, though I didn't realise it at the time, meant he got a place at our best local primary; has he applied in reception he would not have got in as they were oversubscribed then and we are out of catchment).