Florida on Monday became the first state to effectively bar residents under the age of 14 from holding accounts on services like TikTok and Instagram, enacting a strict social media bill that is likely to upend the lives of many young people.

The landmark law, signed by Gov. Ron DeSantis, is one of the more restrictive measures that a state has enacted so far in an escalating nationwide push to insulate young people from potential mental health and safety risks on social media platforms. The statute both prohibits certain social networks from giving accounts to children under 14 and requires the services to terminate accounts that a platform knew or believed belonged to underage users.

It also requires the platforms to obtain a parent’s permission before giving accounts to 14- and 15-year-olds.

In a press conference on Monday, Mr. DeSantis hailed the measure, saying it will help parents navigate “difficult terrain” online. He added that “being buried” in devices all day long was not the best way to grow up.

“Social media harms children in a variety of ways,” Mr. DeSantis said in a statement. The new bill “gives parents a greater ability to protect their children.”

Mr. DeSantis had vetoed a previous bill that would have banned social media accounts for 14- and 15-year-olds even with parental consent. The governor said the earlier bill would impinge on parents’ rights to make decisions about their children’s online activities.

The new Florida measure is almost certain to face constitutional challenges over young people’s rights to freely seek information and companies’ rights to distribute information.

Federal judges in several other states have recently halted less-restrictive online safety laws on free speech grounds in response to lawsuits brought by NetChoice, a tech industry trade group that represents firms including Meta, Snap and TikTok.

Judges in Ohio and Arkansas, for instance, have blocked laws in those states that would require certain social networks to verify users’ ages and obtain a parent’s permission before giving accounts to children under 16 or 18. A federal judge in California has halted a law in that state that would require certain social networks and video game apps to turn on the highest privacy settings by default for minors and turn off by default certain features, like auto-playing videos, for those users.

In addition to social media age restrictions, the new Florida statute requires online pornography services to use age-verification systems to keep minors off their platforms.

Apps like Facebook, Snapchat and Instagram already have policies prohibiting children under the age of 13. That is because the federal Children’s Online Privacy Protection Act requires certain online services to obtain parental permission before collecting personal information — like full names, contact information, locations or selfie photos — from children under 13.

But state regulators say millions of underage children have been able to sign up for social media accounts simply by providing false birth dates.