Relevant part of Coroners and Justice Act 2009 (UK)
Section 65 (regarding what “child” means in the context of indecent images)
(6)Where an image shows a person the image is to be treated as an image of a child if—
(a)the impression conveyed by the image is that the person shown is a child, or
(b)the predominant impression conveyed is that the person shown is a child despite the fact that some of the physical characteristics shown are not those of a child.
(end quote)
In other words, an image can be treated as an indecent image of a child if the “impression conveyed” is that the person is under 18, even if that person has older “physical characteristics”.
This legislation is more directed at non photographic imagery (so hentai / CGI etc) and the reference to physical characteristics is apparently a reference to a large breasts or “1000 year old vampire teeth” not being viable as an excuse that the image doesn’t give the impression of a child.
I can’t recall specifically what legislation was used regarding the age play couple I referenced. I can’t find a specific law that says it’s wrong for a photograph of an adult to appear underage. So it may just be that they were reported to police because they shared their images online without context. I don’t know if they were subsequently prosecuted.
Stone age Thag would be proud…