California is one of only four states with a cutoff date later than Dec. 1.
"We have one-quarter of our school population too young, not doing well unnecessarily. It's a simple fix that impacts millions of California kids in years to come," said Sen. Joe Simitian of Palo Alto.
Under the proposal, roughly 100,000 California kids would have to wait another year.
Research shows beginning school at an older age benefits academic performance and social development.
The state Senate approved the measure, 28 to 4 getting bipartisan support. All the opponents were Democrats.
"As a parent, I want my children to start kindergarten early. I think, as a society, we want them to get more education, not less," said Sen. Lois Wolk of Davis.
The state will save about $700 million a year from having fewer children in kindergarten. Half would go toward increasing preschool enrollment, and the other half would help alleviate the budget deficit.
That extra money won't help all families. By delaying the entry age, some parents worry about having to pay for daycare or preschool for another year.
Still, some teachers insist the move is good.
"That would definitely be more beneficial to them, if they spend another year in preschool getting those kindergarten readiness skills together," said Dana Rivers, a teacher from Hawthorne.
"The class keeps moving, and that child that's not ready falls behind," said Melissa Ali, a teacher from Carson.
For quick learners, parents would still be able to ask their school districts to let their 4-year-old start kindergarten.
The proposal now heads to the Assembly.
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