Under-16s in Australia have been banned from using major social media services including Tiktok, X, Facebook, Instagram, YouTube, Snapchat and Threads.
They cannot set up new accounts and their existing profiles were deactivated.
The ban is the first of its kind and has been watched closely by other countries.
Australia's government said the ban would reduce the negative impact of social media's "design features that encourage [young people] to spend more time on screens, while also serving up content that can harm their health and wellbeing".
A study it commissioned in 2025 found that 96% of children aged 10-15 used social media, and that seven out of 10 of them had been exposed to harmful content. This included misogynistic and violent material as well as content promoting eating disorders and suicide.
One in seven also reported experiencing grooming-type behaviour from adults or older children, and more than half said they had been the victim of cyberbullying.
Ten platforms are currently included: Facebook, Instagram, Snapchat, Threads, TikTok, X, YouTube, Reddit and streaming platforms Kick and Twitch.
The government assesses potential sites against three main criteria:
YouTube Kids, Google Classroom and WhatsApp have not been included as they were not deemed to meet those criteria.
Under-16s are also still able to view most content on online platforms that do not require an account.
Critics have called on the government to extend the ban to cover online gaming sites like Roblox and Discord, which are not currently included.
In November 2025, Roblox said it would introduce age checks on some features.
Children and parents are not punished for infringing the ban.
Instead, social media companies face fines of up to A$49.5m (US$32m, £25m) for serious or repeated breaches.
The government says firms must take "reasonable steps" to keep kids off their platforms, and should use multiple age assurance technologies.
These could include government IDs, face or voice recognition, or so-called "age inference", which analyses online behaviour and interactions to estimate a person's age.
Platforms cannot rely on users self-certifying or parents vouching for their children.
Meta, which owns Facebook, Instagram and Threads, began closing teen accounts from 4 December last year. It said anyone mistakenly kicked off could use government ID or provide a video selfie to prove their age.
Meta said it blocked approximately 550,000 accounts during the first days of the ban.
Snapchat said users could use bank accounts, photo ID or selfies for verification.
Initially, there were fears that age assurance technologies may wrongly block adults while failing to spot underage users.
The government's own report found that facial assessment technology is least reliable for teenagers.
Questions were also raised about the scale of the potential fines.
"It takes Meta about an hour and 52 minutes to make A$50 million in revenue," former Facebook executive Stephen Scheeler told the AAP news agency.
Critics have also argued that the limited scope of the ban - even if properly implemented - undermined its ability to protect children.
Dating websites have been excluded, along with gaming platforms and AI chatbots, which have recently made headlines for allegedly encouraging children to kill themselves and for having "sensual" conversations with minors.
Others think that educating children about how to navigate social media would be more effective.
Some teens told the BBC they had set up fake profiles ahead of the deadline - although the government warned social media companies to identify and remove such accounts. Others switched to joint accounts with their parents.
Critics have also raised concerns about the large-scale collection and storage of data required to verify users' ages.
Australia - like much of the world - has had a series of high-profile data breaches where sensitive personal information was stolen and published or sold.
But the government insists the legislation incorporates "strong protections" for personal data.
These stipulate that it may only be used for age verification and must be subsequently destroyed, with "serious penalties" for breaches.
Social media companies were aghast when the ban was first announced in November 2024.
Firms argued it would be difficult to implement, easy to circumvent and time consuming for users, and would pose risks to their privacy.
Companies also suggested it might drive children into dark corners of the internet and deprive young people of social contact.
Snap - which owns Snapchat - and YouTube also denied being social media companies.
Days before the ban was due to take effect, YouTube said the "rushed" new laws would leave children less safe as they would still be able to use the platform without an account, removing "the very parental controls and safety filters built to protect them".
YouTube's parent company, Google, reportedly considered a legal challenge over its inclusion but did not respond to a BBC request for comment.

Though included in the ban, YouTube has denied being a social media company
Despite implementing the ban early, Meta warned it would leave teens with "inconsistent protections across the many apps they use".
At parliamentary hearings in October 2025, TikTok and Snap said they opposed the ban but would follow it.
Kick - the only Australian company covered by the new law - said it would introduce a "range of measures" as it continued to engage "constructively" with authorities.
A day ahead of the ban Reddit said it would comply but that it had "deep concerns" about the law which "undermines everyone's right to free expression and privacy".
In the days before the ban started on 10 December, thousands of Australians sought lookalike apps to fill the void, with three little-known apps - Lemon8, Yope and Coverstar - surging in downloads. But that initial rise has since dropped.
The number of Australians downloading virtual private networks – or VPNs – also increased before the ban, but has since fallen back to normal levels.
VPN technology allows users to hide their location and pretend they are based in another country, in effect, bypassing local laws.
Teenagers the BBC spoke to a month into the ban had mixed feelings about it. One said she "felt free", while another said it hadn't "really changed anything", and that he was spending about as much time on social media platforms as before the ban, in some cases by using accounts with fake birthdays.
Denmark has announced plans to ban social media for under-15s, while Norway is considering a similar proposal.
A French parliamentary enquiry has also recommended banning under-15s from social media, and a social media "curfew" for 15- to 18-year-olds.
The Spanish government has drafted a law which would require legal guardians to authorise access for under-16s.
In the UK, new safety rules introduced in July 2025 mean online companies face large fines or even the jailing of their executives if they fail to implement measures to protect young people from seeing illegal and harmful content.
In January, the House of Lords voted to support a ban of under-16s from social media platforms, in an amendment to the government's schools bill.
Meanwhile, an attempt in the US state of Utah to ban under-18s from social media without parental consent was blocked by a federal judge in 2024.