If she does not have a diagnosed delay (where an extra year for tutoring is needed), it is your responsibility to send her on her way. Holding her back is just slowing down the things she should be learning at this ripe age. It would be a disservice to her!
We live in a competitive, selfish, dog eat dog world. Don't push your kids into that by holding them back. The example is wrong. And, what is next? The older kids have higher expectations? What if they can't fulfill them? Or, they start out knowing they are older and feel they were held back because they were inadequate? What consequences will there be then?
If it were statistically proven that our kids would thrive better by waiting a year, then Kindergarten would start at 6, not 5. Maturity is experience. And, kids get that from home and peers. It is petty to hold back a child because of a presumed advantage in sports or early ability to drive. Furthermore, studies have found that there is no correlation between self esteem and stature. Finally, struggle is good for kids too. It teaches them that they have to work for what they get, which is a valuable life lesson.
I am sickened by the fearful parents who hold their normal kids back because of what might be or, how their child might compare to others. In my opinion, they send the wrong message to their kid, risking more in the end.
By the way, both my kids have summer birthdays along with about 1/4th of the population. So, your kids won't be alone.