Not disallowed, just they also have to offer an alternative.

Question is why hasn't Australia created a Digital ID system that can prove you're >= 16 years old without giving away other info?

In the spirit of "Falsehoods programmers believe in"[1] for human ages:

* Not all people know their age.[2]

* Even if people do know their age they may not have any means to prove their age.[2]

* Even if people know their age, they may know their age only in a calendar system which is ambiguous or with a margin of error.[3]

* Even if people have documentation proving their age, the documentation may provide an approximate age or use a calendar system which is ambiguous or with a margin of error.[3]

* Even if people have documentation proving their age, they may know it to be incorrect.

* People may have multiple documents each nominating a different age.

* People may be reissued with new documents changing their recognised age.

* Even if the government tries to guess someone's unknown age, it's an inexact science and could be revised later.

[1] https://github.com/kdeldycke/awesome-falsehood

[2] https://www.racgp.org.au/getattachment/fe71891a-aafe-453f-a3...

[3] Example calendar: https://en.wikipedia.org/wiki/Igbo_calendar