My son has a friend in his class who's birthday is September 1st 1999 He also has a girl in his class wh's birthday is 31st August 2000. They are just about a whole year apart yet in the same class. Cut off is very definately 31st August.
The children start the year they're about to turn five. School year runs from September to July. In some schools they stagger the entry times for children so a child that is 5 in the months Sept, Oct, Nov or Dec. will start in September. Children with birthdays in months from January- August will start in the January term.