I agree, at 18 months you can't really say how ready your DD will be when she starts school at 4. Some seem absolutely fine and do well, whereas others don't have a clue what's happening and would be a lot better off starting a year later, especially boys.
Research does generally suggest that summer born DC don't do as well as autumn born DC throughout their education, so it is better to be a bit older when starting.
Personally, I am still upset with the inflexibility of the English system which only seems to serve itself but not the individual DC. DS (April birthday and now in Y3) would really have benefited from starting later, he is not summer born, but very immature. It still shows, as he prefers to play with younger DC, he hates reading, writing and maths (despite being average) and generally doesn't take responsibility for HW or anything else. We were hoping he will be OK, but I believe he would have had a better chance starting later - he might even enjoy school now if he hadn't been pushed into formal learning too early.
On the other hand, DD (just 4) will be more than ready next September. She concentrates well, loves being read too, attempts to read and write, sits for ages and draws - her nursery said that they think she will do very well at school. A completely different child who would cope well if she had to start now.