In addition to the AVS fine, Ofcom also announced that one "major social media company" was going through compliance remediation with its enforcement team.
The regulator has not named the platform but says there may be formal action if it does not see sufficient improvement soon.
Ofcom said the fine showed the "tide on online safety" was beginning to turn.
"This year has seen important changes for people, with new measures across many sites and apps now better protecting children from harmful content," said Oliver Griffiths, Ofcom's online safety group director.
"But we need to see much more from tech companies next year and we'll use our full powers if they fall short," he added.
Ofcom has already started issuing fines to some companies for not implementing proper age verification, including deepfake "nudify" applications.
However, online message board 4Chan has so far refused to comply with a £20,000 fine issued by Ofcom over the summer.