This is a non-issue because it's almost certainly going to be gated behind a permission prompt. There are more invasive things sites/apps can ask for, and we seem to be doing fine, eg. location. Moreover is it really that much of a privacy loss if you go on steam, it asks you to verify you're over 18, and the OS says you're actually over 18?
>Biometric data? Today it unlocks your private key. Tomorrow it's used to verify you are the same person that was used during sign-up -- the same that was "age-verified".
Given touch id was introduced over a decade ago, and the associated doom-mongering predilections did not come to pass, I think it's fair to conclude it's a dud.