I wonder if the instructions are consistent enough that someone could make a bespoke video on YouTube that opens its mouth at the right time.

tbh it looks like most of these platforms are going to be predominantly relying on historical analysis and account age, so most people probably won’t ever need to provide any evidence. If your Facebook account is 10+ years old, has photos of you on it already, and you’ve shared content including boring adult subjects like politics and professional sports, they’ll probably already assume you’re an adult.

I’ve definitely been getting increasingly frustrated at Facebook’s algorithm though. At the very least I am seriously considering installing an extension that hides the entire News Feed, so I only see stuff that I specifically seek out, which will likely only be on a handful of pages I actually care about.