Washington, Jan. 26 -- Meta Platforms, TikTok and YouTube will face courtroom scrutiny this week over allegations that their platforms are fueling a youth mental health crisis, as the national debate about kids' screen time enters a new phase.

The bellwether trial in California Superior Court, Los Angeles County involves a 19-year-old woman from California, identified as K.G.M., who says she became addicted to the companies' platforms at a young age because of their attention-grabbing design, according to court filings. She alleges the apps fueled her depression and suicidal thoughts and is seeking to hold the companies liable.

Her lawsuit is the first of several cases expected to go to trial this year that center on what the plaintiffs...