From
https://www.qantas.com/us/en/frequen...ub/access.html :
Customers under the legal drinking age must be accompanied by an adult in any Qantas operated lounge.
As the lounge staff implied, this will be a part of their liquor license, not just a Qantas policy. Both the passenger and Qantas can be liable to fines, up to and including the loss of their license.
It's also stated in the
T&Cs for the Qantas Club :
10.11 Children under the age of 18 years must be accompanied by an adult when visiting any lounge.
10.12 Customers under the legal drinking age must be accompanied by an adult in any Qantas operated lounge. Legal drinking age may vary in different jurisdictions and countries.