When my stepson was in preschool, his classmates' birthdays ranged from July to November. Only two students from that class waited to start K -- one was the son of a teacher (August birthday, she recognized that her son was not as mature as his older sibling had been when entering K) and my stepson, whose birthday is mid-September, but his mom's district allowed no exceptions to a Sept. 1st cut-off. His classmates who turned 5 in October and November of that year all thrived in kindergarten and many are now in magnet high schools, the local youth symphony (highly competitive), and fared very well in writing, math, and chess championships with kids a full year older.
My own kids, who both have summer birthdays, started kindergarten at age 4 and have done really well -- although previous posters have made good points about issues such as handwriting that tend to plague students who are young for their grade. They have had no issues with academics, academic competitions, standardized testing, or fitting in with outside classes with kids who may be a full year older than they are.
I have no issue with choosing to retain a child who is not ready, but I did want you to know that choosing to move forward can be successful.