No, "digital credentials" is an awful idea because it requires to store your ID on your phone and thus make it accessible to Apple and Google and secret courts. What I suggest is simply to store a single "isAdult" bit on device, without revealing any identity, and make apps like browser do the censorship on device, without sending any data to a webite. The algorithm is as follows:

    if isAdult == 0 and website doesn't send a "safe-content" header, then:
        browser refuses to display content
    if isAdult == 0 and photo in a messenger doesn't contain a "safe-content" metadata, then
        photo viewer refuses to display content
    if isAdult == 0 and the app is not marked as safe, then
        app store refuses to download the app and OS refuses to launch it

With my approach, you don't need to store your ID on your device, you don't need to send your ID anywhere, and website operators and app developers do not need to do anything because by default they will be considered not safe. So my solution's cost is ZERO for website operators and app developers. As a website operator you don't need to change anything and to verify the age.