The "app" could be a good solution, if it didn't require attested Android or iOS. It could, for example, have me plug my ID chip into my GNU/Linux system and expose it with a standard protocol. That would be no problem. The problem is that they do not want such a way.
In any case, I think that age gating would not be needed if the platforms were regulated to remove addictive recommendation algorithms.
ID chip? Is that something everybody in the EU (or whatever region) has? Is it just embedded in your driver license or passport?
[I'm in the US, we're very ID-averse here, weird, but is what it is]
Using an ID card reader is already possible. See here for a list of Linux repos supporting German IDs: https://www.ausweisapp.bund.de/en/open-source Finding a working hardware/software combination for your ID card is up to you.
The app is an alternative for people who don't want to buy or carry around a card reader, but who already have a smartphone.