As an educator, I can say it is really about each individual kid and not the age. Considering your child has spend two years in preschool she is most likely ready. I can tell you that my daughter has a June birthday (but a year behind yours) and I am already planning on putting her into kindergarten at 5. There are individual cases where some kids are not socially or academically ready but if your daughter knows her letters and the sounds they make and can recognize her name, you are in good shape. If she can already read, please don't hesitate to put her in school. Honestly, it might be worse to wait a year and have her be bored in class because she is not being challenged.
And sadly, Diane is right. In most cases where the parents hold kids back (or start them later) it's for a nonacademic reason (I know someone who did this).
I say ignore the naysayers and do what your originally planned! Send her to school!