A Bill which has just been passed by Parliament allows schools to enrol children before their fifth birthday.
They can enrol them in groups, or cohorts, at the beginning of each term, with children able to start school from the beginning of the term closest to their fifth birthday.
Until now their fifth birthday determined when they would start school, and Education Minister Nikki Kaye says the system needs to be flexible.
When children younger than five start school will be up to their parents, and the youngest a child can start is four years and 10 months.
Ms Kaye said some schools were already encouraging early starts.
"These schools believe cohort entry enables them to support a better transition into school, simplifies school and classroom planning, and minimises disruption to existing students," she said during the third reading debate on the Education (Update) Amendment Bill.
"Some concerns have been raised that schools will be starting children too early, but the important part of this enabling legislation is that it ensures parents have the choice about the right time for their child to start school."
Labour leader Andrew Little doesn't agree with earlier starts.
"At the age of four you should be running around in a sandpit and the garden making cups of tea and doing whatever with muddy water," he said earlier this week.
"Kids should be doing that stuff, exploring their creative and fun side."
The Bill contains other changes to the education system, and Ms Kaye said it was the most significant update for 30 years.
It makes clear that seclusion - isolating misbehaving children - isn't allowed.
'It sends a clear signal to educators that in today's world there is no situation where it is acceptable for children and young people to be secluded," she said.