Meta Platforms Inc. warned of significant difficulties identifying and removing underage users as it prepares to comply with Australia’s world-first social media ban for under-16s next month.The owner of Facebook and Instagram said on Thursday it would begin notifying underage users, adding it planned to start removing accounts on Dec. 4 ahead of the government’s Dec. 10 deadline. But it cautioned that there remained a “significant” margin of error when determining whether a user is 16 years or ...