My son is born in July. I have been considering waiting a year to send him to school. His dad was also born in July and his parents waited. I think it is going to depend on how advanced we feel he is. My sister is a July baby too, but she went right away and was the youngest in her class. She was the president of several different organizations and graduated as valedictorian of her class in high school. She graduated from college with honors in 3 years. She now holds a great job and continues to move up. I really think that it depends on a variety of factors. If my son doesn't seem ready, then we probably will wait a year. But if he is quick to learn, then I will definitely send him to school when it is time.
I think the reason a correlation could be made here has more to do with the parents' eagerness, rather than the kid's birthday. Say you have to be 5 by August 1st in order to start kindergarten. So the child's birthday is July 31st, so they are the very minimum age. And they act it; they are still in the mindframe of a 4 year old. But the parents start the child in kindergarten anyway. This may affect their outcome.
I went to kindergarten twice. After my first year, my parents had me evaluated to see where I was, and they determined that I should spend another year in kindergarten so that I wasn't overwhelmed when I got to first grade. That was a great decision on their part, if you look at my grades and accomplishments. I plan on doing the same with my kids. Sending them to school as I feel they are ready, not necessarily based on their age or when their birthday is.