To make sure we're all on the same page, this proposal involves creating an account with a service provider, then uploading some sort of preexisting, established proof-of-identity (eg passport data page), and then requesting a token against that account. The token is timestamped and non-fungible, so that when the token is presented to an age-restricted website, that website can query the service provider to verify that: 1) the token is still valid, 2) the person associated with the token is at least a certain age.

If I understood that correctly, what you're describing is an account service combined with an identity service, which could achieve the objectives of a proof-of-age service, but does not minimize privacy complications. And we already have account services of varying degrees and complexity: Google Accounts, OAuth, etc. Basically any service where you log-in, since the point of logging in is to associate to a account, although one person can have multiple accounts. Passing around tokens isn't strictly necessary since you can just ask the user to prove account ownership by signing into their Google Account, for example. An account service need not necessarily verify age, eg signing in to post a comment on a news article.

Compare this with an identity service like ID.me, which provide records on an individual; there cannot be multiple records for the same live person. This type of service is distinct from an account service, but some accounts are necessarily tied to a single identity, such as online banking. But apart from KYC regulations or filing one's taxes online, an identity service isn't required for most day to day activities, and any additional uses pose identify theft concerns.

Proof-of-age -- as I understand it from the Australian legislation -- does not necessarily demand an identity service be used to satisfy the law, but the question in this Lemmy thread is whether that's a distinction without a difference. We don't want to be checking identities if we don't have to, for privacy and identity theft reasons.

In short, can a person be uniquely, anonymously age-verified online? I suspect not. Your proposal might be reasonable for an identity service, but does not move us further towards a theoretical privacy-centric proof-of-age validation mechanism. If such a mechanism doesn't exist, then the Australian legislation would be mandating identity checks for subject websites, which then become targets for the holder of those identity records. This would be bad.