Written erotica does not require age verification in the UK under the Online Safety Act.
However, pornographic images, including those generated by AI, would require users to prove they are over 18.
In the US, critics say OpenAI's decision to allow erotica on the platform shows the need for more regulation at the federal and state levels.
"How are they going to make sure that children are not able to access the portions of ChatGPT that are adult-only and provide erotica?" said Jenny Kim, a partner at the law firm Boies Schiller Flexner.
Ms Kim is involved in a lawsuit against Meta that claims the company's Instagram's algorithm harms the mental health of teen users.
"OpenAI, like most of big tech in this space, is just using people like guinea pigs," she said.
In April, TechCrunch reported that OpenAI was allowing accounts in which a user had registered as a minor to generate graphic erotica.
OpenAI said at the time that the company was rolling out a fix to limit such content.
A survey published this month by the nonprofit Centre for Democracy and Technology (CDT) found that one in five students report that they or someone they know has had a romantic relationship with AI.
On Monday, California Governor Gavin Newsom vetoed a bill passed by the state legislature that would have blocked developers from offering AI chatbots companions to children unless the companies could guarantee the software wouldn't breed harmful behaviour.
Newsom said it was "imperative that adolescents learn how to safely interact with AI systems" in a message that accompanied his veto.
At the nationwide level, the US Federal Trade Commission (FTC) has launched an inquiry into how AI chatbots interact with children.
In the US Senate last month, bipartisan legislation was introduced that would chatbot users to file liability claims against their developers.
