Three problems with this:

1. Removes the pain of age verification, encouraging some people to stay in the proprietary walled garden when everyone would be better served by open platforms (and network effects).

2. Provides a pretext for more invasive age verification and identification, because "the privacy-respecting way is too easily circumvented".

3. Encourages people to run arbitrary code from a random Web site in connection with their accounts, which is bad practice, even if this one isn't malware and is fully secure.