The biggest problem with conspiracy theories like this is always the number of people involved keeping their mouths shut. Anyone that has ever managed a large project knows how impossible it is to keep a large group of people quiet about something. In real life, there are conspiracies. Often very large ones. But they didn't stay secret for long. What is easier to believe: (1) that all these people involved, across countries with leaders of many different political varieties, all agreed to stick to a single narrative in order to cover up a deep international conspiracy to build a massive international database of people's ages online, OR (2) Meta and other orgs are doing a normal business thing and trying to reduce their liability costs.