Enlistment

There is an entry in the goofs that says
In the movie it is stated that a person had to be 21 years old to enlist. According to the Australian War Memorial website for enlistment, in 1914 the age requirements were 18-35 years old, and in 1915 the age requirements were 18-45 years old.
My recollection (from reading - not personal experience) is that you could sign up with parental permission between 18 and 21 and then without permission from 21 onwards.
My recollection from the film is that the uncle didn't actually say you need to be 21 but that they wouldn't accept him until he was 21. This is because Archie's uncle had said that his parents would never agree.
So as far as Archie was concerned he couldn't enlist until he was 21 and it is not a goof.
Is anyone else able to confirm this?

"They who... give up... liberty to obtain... safety, deserve neither liberty nor safety."