Experts fear kids are going to circumvent the ban with relative ease - either by tricking the technology that's performing the age checks, or by finding other, potentially less safe, places on the net to gather.
And backed by some mental health advocates, many children have argued it robs young people of connection - particularly those from LGBTQ+, neurodivergent or rural communities - and will leave them less equipped to tackle the realities of life on the web.
But the policy is wildly popular with parents and has won the support of people like talk show host Oprah Winfrey, and Prince Harry and Meghan, the Duchess of Sussex.
In a statement on their website, the pair lauded the "bold" action from Australia but said "it shouldn't have come to this".
"We hope this ban is only the start of a reckoning between society and the tech companies that built these platforms with growth as their first principle instead of safety."
Various governments, from the US state of Florida to the European Union, have been experimenting with limiting children's use of social media. But, along with a higher age limit of 16, Australia is the first jurisdiction to deny an exemption for parental approval in a policy like this - making its laws the world's strictest.
Reddit said the law forces "intrusive and potentially insecure verification processes on adults as well as minors", isolates teens engaging in "age-appropriate community experiences" and creates an "illogical patchwork of which platforms are included and which aren't".
"There are more targeted, privacy-preserving measures to protect young people online without resorting to blanket bans."
The case is not "an attempt to avoid compliance" or "an effort to retain young users for business reasons", it added.
"Unlike other platforms included under this law, the vast majority of Redditors are adults, we don't market or target advertising to children under 18," it said.
The other platforms affected by the ban include Facebook, Instagram, Snapchat and TikTok.
