It implies that the user has access to the technical infrastructure that supports age verification. Sucks to be you, if you can't afford a recent Apple or Android device to run the AgeVerification app.
There is also the problem of mission creep. Once the infrastructure is in place, to control access to age-restricted content, other services might become out of reach. In particular, anonymous usage of online forums might no longer be possible.
The EU Digital Wallet requires hardware attestation so only it only works on locked-down government-approved OSes. That opens the door for government control of all electronic devices.
There is also the problem of mission creep. Once the infrastructure is in place, to control access to age-restricted content, other services might become out of reach. In particular, anonymous usage of online forums might no longer be possible.