First, a Slate article about education and middle upper class snobbery. My response.
Exactly. Case by case. Case one in my class this year, November birthday. Academically ok, but could do better. Probably on the quieter end of the spectrum to begin with, but cried at the drop of a hat with any wrong answers. Irresponsible to a fault, she never remembered to turn in her homework or school communication with her mother.
Same class, a late August birthday. One of the most mature and responsible students in my class. I'd venture to guess that more than age had something to do with this disparity between these two second graders. Stark contrast in home lives for sure.
The big difference between some parents is not whether they send their kids to school early. It's how the parents view school. Fact is (sadly) some parents only view school as free babysitting and unwittingly convey that to their kids. The kids come to view school as unimportant and don't try as hard or do as well. A little fatalistic perhaps, but each teacher only has each child for one school year, six hours (or so) a day. It's the parents' responsibility to reinforce school's lessons and nurture a love of learning that will ensure their success.