It depends on the area, but I know a lot of school districts will NOT allow early entrance and will make the child follow their date requirements unless the child has completed 1st grade.
Food for thought...
I started Kindergarten at 4, and while I was fine academically and socially, it was obvious that I was not emotionally mature enough by Jr. High and High School. It definitely would have been beneficial for me to have waited another year.
My oldest missed the kindergarten cut-off by a month and we considered requesting an early admittance, but I am SO glad we didn't. He, too, struggled emotionally despite being almost six, academically way ahead of Kindergarten curriculum, and making friends easily. He enjoyed school but had a lot of issues with handling stressful situations (bus rides with older, louder kids; changes in routine, substitute teachers, etc.). It would have been a complete disaster if he had started earlier.