LOS ANGELES — Mark Zuckerberg expressed regret over Meta’s slow rollout of tools to spot children under 13 on Instagram. Testifying under oath for the first time before a jury, the 41-year-old CEO fielded tough questions from lawyers representing a young California woman who blames social media for her mental health struggles.

The trial, unfolding in a federal courtroom here, centers on Kaley G.M., a 20-year-old who began using YouTube at age six, Instagram at nine, and later TikTok and Snapchat. Lawyers accuse Meta and Google-owned YouTube of designing addictive features that fueled her depression and anxiety. The case, the first of many against social media giants, could set precedents for thousands of similar suits filed by U.S. families.

Zuckerberg started reserved, according to observers in the courtroom. He grew animated as plaintiff attorney Mark Lanier hammered him on internal emails. One from 2015 showed Instagram had four million users under 13 that year — when Kaley signed up — and that 30% of U.S. children aged 10 to 12 already used the app.

“I always wish that we could have gotten there sooner,” Zuckerberg said when pressed on complaints that age checks fell short. He insisted Meta has improved and now occupies “the right place” on verification. Still, Lanier cited an email from former policy chief Nick Clegg calling it “indefensible” that Instagram claimed to bar under-13s without enforcing it.

Lawyers confronted Zuckerberg with documents revealing past company goals to boost time spent on Instagram. He acknowledged those targets but stressed Meta’s mission to build “useful services” that connect people. Time on the app, he said during questioning by his own attorneys, amounts to a “side effect” of good experiences. He often turned to the 12 jurors, gesturing emphatically to drive points home.

Zuckerberg pushed responsibility onto Apple and Google. Those firms control smartphone operating systems used by billions, he argued, and should handle age verification at the device level. “Doing it at the level of the phone is just a lot clearer than having every single app out there have to do this separately,” he said. “It would be pretty easy for them.”

The Meta head, whose company also runs Facebook and WhatsApp, shook his head and waved hands in frustration at times. Lanier highlighted Zuckerberg’s prior congressional testimony denying efforts to maximize user time — a claim undercut by the emails. The CEO conceded Meta once tracked such metrics but framed them around enhancing engagement.

Plaintiffs seek to prove Meta and YouTube knowingly fostered compulsive habits in youth, contributing to a surge in teen mental health crises like anxiety, eating disorders and suicides. TikTok and Snapchat settled with Kaley before trial. Proceedings against Meta and Google run through late March, when jurors will weigh liability.

This bellwether case draws intense scrutiny. Families nationwide have filed suits pinning youth mental health epidemics on social platforms. A verdict here could reshape how tech firms approach young users — or trigger massive payouts.