Hi O.,
I have twin boys who are 7, they have mid June birthdays. One of my twins I felt was a little more ready but mostly for social reasons. We had actually been debating for two years prior to kindergarten about whether to start them as 5 year olds because they would have been young like your kiddos. After talking to many many people who had gone before us we made the choice to give both boys an extra year, they did pre-k again in an Abeka curriculum school....BEST decision we could have made. When they started kindergarten as six year olds they were so very ready. Their teacher raved constantly about how prepared they were academically, mentally and socially. It was a wonderful year. Now they are in first grade and the success continues. Their teacher tells me they are so much more mature than most of the other boys (which I think makes life a little easier for her every day! LOL).
On the other hand, I have a friend at church whose son is two months younger than my boys (August 27th birthday) and they started him as a young five year old and it has been a difficult couple of years (he is a second grader now). Socially and academically, Luke has struggled and probably will continue. When he has to start taking the high pressure standardized test in third grade next year, I wonder how well he will do.
I know each case is different, there are some I've seen here on MP that are passionate about not holding kids back and some that think it's fine. You know your kids best. I look at it like this...you are a lot more likely to have regret sending a child you pretty well know is not ready than to have regret giving an extra year to mature up. Holding back worked wonderful for us.
Like others have said though, do not send one twin and hold the other back, I can forsee all kinds of bad consequences for that in the future.
Best wishes on your decision! Please let me know if I can share anymore about our experience with you :-)