It really depends on the child... I have 2 girls with August birthdays... one we did not retain at that point, and one we did.... each girl was totally different.
The older one we sent to Kindergarten when she was 5.... she was really smart, but had a hard time focusing... there was just so much more that was interesting to her. In second grade, her teacher recommended having her repeat 2nd grade, but we declined. At 4th grade, she kind of hit a wall.... the odd thing was, when they did the Iowa Basics tests that year, she tested in math computation, she tested at a late 3rd grade... but for math comprehension, she tested at 7th grade! She intuitively understood it, but at that point, had not memorized her multiplication and division facts, and really struggled.
That was also a rough year for us as a family... too many things to go into right now.... but we moved at the end of the school year to Texas (from Iowa), and had her repeat 4th grade at that point. For her, that was the best thing we could have done..... it gave her a chance to catch up both socially and academically.... she just finished her Doctorate in Microbiology at Johns Hopkins, to let you know how that ultimately did help her.
The other August birthday, was always different from her big sisters.... she just wasn't ready to sit down and learn. We delayed her entry into Kindergarten, and had her attend a different pre-school program. It did help some, but she still struggled when she got to middle school and high school... she is really bright, but was having social problems, and some academic problems. We now realize that she is ADHD, and struggled with focus and social issues all those years....
What I'm trying to say is that we really can't tell you WHAT to do...all you can do is do the best you can to make what you feel is the right decision at the time..... would daughter #1 have turned out differently if we had delayed her entry into Kindergarten? I don't really think so...... but we had her repeat a grade at pretty much the right time for her, and it worked out just fine.
We would have had problems with the other daughter no matter what we had done..... we just had to deal with it the best we could.