The trend nationally has been to let children linger on their parents' health insurance plans. Former President Barack Obama's health care law let children stay on their parents' plans until age 26. Some states have gone further and let kids stay on their parents' plans until at least age 30, including Florida, Illinois, Pennsylvania and New Jersey.

But California is now the first state to go the other direction by letting some adults join their kids' health insurance plans. Gov. Gavin Newsom, a Democrat, signed the law this week, but it won't take effect until 2023.

"The signing of the Parent Healthcare Act will help more families care for their parents the way they cared for us," Insurance Commissioner Ricardo Lara said.

To be eligible, adults must rely on their child for at least 50% of their total support. The law applies only to people who buy their health insurance on the individual market. Those who get insurance through their jobs, which includes most people in the state, aren't eligible.

That makes the law much cheaper. A previous version, which would have applied to more people, could have increased employer premiums between $200 million and $800 million per year, depending on how many people enrolled. That prompted business groups, including the California Chamber of Commerce, to oppose the bill — winning key concessions.