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

Does that matter though? At some point someone's assigning them a legal age so you can decide if they're over 16 or under 18, or over 21 etc. Your real age doesn't matter. And why would it not be able to be corrected later? Another advantage of a digital ID