63D outlines the requirement:
A provider of an age-restricted social media platform must take reasonable steps to prevent age-restricted users having accounts with the age-restricted social media platform.
There isn't really a definition of what reasonable steps are but that looks like it's on the commissioner to define them under section 27 of the act. A platform is a service that basically allows interaction between users and a service "includes a website" (does that include gaming servers?) so Lemmy sites would most certainly fit the bill. It really depends how onerous these reasonable steps are as to whether Lemmy sites will be able to implement them.
Not sure how the act implementers plan to deal with servers hosted in other countries. Will they block them? If not then this is mostly a paper tiger, but also an impediment to further development of Australia based platforms.
I remember reading an article once that claimed that 12 year old girls had some of the most buying power in western societies due to their influence on their fathers decision making. It suggested that there were whole ad campaigns focused on their demographic for a huge range of things that you wouldn't normally associate with 12 year old girls (eg cars).
The legislation does appear to include requirements for handling of personal data acquired to confirm age of users.