I would suggest that you do not push ahead too quickly. I am an early childhood consultant for the state of LA and am the mother of 5 children ages 33-14. Two of my children had Sept. birthdays and one had a Nov. birthday. I kept the Nov. son back a year and it was the smartest thing I have ever done. He was mature enough to make excellent decisions concerning his career and at age 27 he is very successful. Whereas, my 33 year old son, whom I did not hold back, is still floundering with his career. My 16 year old daughter in now a Junior in High School and is struggling due to her immaturity in some aspects of her life. Her prek teacher said she was ready for Kindergarten so I put her in at age 4 (She turned 5 before the cut off of Sept 30 by 7 days) and it was the biggest mistake of her life. She, like my oldest son, has struggled in school since 3rd grade. That is the year the age seems to catch up with them. They are both capable academically but their immature attitudes have kept them from being successful. My 14 year old daughter who has a May birthday is moving on ahead quite successfully as well. No matter what anyone says, maturity, just by a few months, makes all the difference.
I read an article at some point that made a lot of sense. It stated that such a decision should be based on all areas of the child's development - physical, emotional, social and academic maturity. My 16 year old was ready socially and academically to start Kindergarten early, however, physically she was not able to handle the demands of the day. As a Junior, she still has trouble meeting the demands of the day. She was a very tiny infant and had a soft trachea that delayed her physical activity for the first six months of her life. She is fine now physically but that six month delay effected all of her motor development needed in learning to read and write. So my advice to you is to take a very, very close look at your child in all areas and be sure she is ready to move forward. If there is the slightest doubt in even one area, I would keep her back a year. As one of the other respondents said, one more year of maturity does not ever hurt. Picture her graduating at age 17 - will she be ready to make the necessary decisions for her life. I graduated at age 17 and was very mature and level headed but I wish I had had an extra year of growth. Maybe then I would not have made as many mistakes in my decisions for my life as I did.
Preschool has proven to be a very effective measure of success in our children here in LA. Those children attending preschool score much higher on standardized tests at the end of fourth grade than those students who did not attend preschool. So, yes, put your child in a good program, but make sure it is right for her. Let's let our children be children.