Research shows it is not really about age and being developmentally ready age wise, it's about exposure. You become 'developmentally ready'' when you have been exposed to the skills necessary to learn the next thing. Age is not the most relevant factor
I think this is a rather simplistic way of looking at child development.
I know one anecdote does not make evidence, but, to give an example: ds did not talk coherently until he was 3+ but did speak a fluent form of serbo-croatian
I did not worry as I knew that I apparently didn't talk until I was 3 (and ds had the added disadvantage of being male and I'd read that boys are often later speakers than girls). Ds is now 11 and I wish he would shut up speaks fluently and argues with us articulately.
He was surrounded by books, read to, watched Baby Einstein DVDs, had phonics posters etc etc from birth. He started school (just before his 5th birthday, in the younger half of the year) able to recognise the letter M and that was all. He still wasn't ready to read aged 5. He was ready age 6 (and 3/4) towards the end of P2. This was not a case of the school not teaching him properly.
He left the school this summer as one of the brightest in his class, with the teachers saying lots of complimentary things about him and all saying how ready he was for secondary and how well he would do there.
But even if he weren't bright (because everyone is not the same), the best thing they have done for him is to help ensure that he loves reading. He writes stories of his own accord at home. He reads in his free time. What more can you ask for? :)