Can you provide some evidence that it didn’t work in Australia? Given the ban hasn’t been in place that long I’d like to see your sources about it not working.
Limited success might be a better term. But if a supposedly blanket ban only stopped 30% of under-16s accounts from accessing social media, that does seem pretty failure-esque.
Presumably you wouldn't call laws against murder a failure because there are murders.
Murder is not popular. Murderers are thankfully rare, and they don’t comply with the laws on murder. The masses do.
When you make popular things illegal, you erode respect for the law and turn people against the government. You can (maybe) get away with this when times are good, but when times are not good you are doubling your problems. There is a spillover into other kinds of criminality as law-abiding people decide that the law may not matter as much as it once did. Ignore this at your own peril.