Where does it say "..a large proportion of those going under the knife are under 18"?
The story I read following the archive link says
"A series of freedom of information requests show that 18-year-olds were among those going under the knife..."
"Young people can be referred for surgery from the age of 17, though the operation cannot take place until they are 18."
"Whilst a detailed breakdown of ages was not released, the data show that in 2021, of the 189 procedures carried out, 47 per cent of the patients were under 30 and the youngest was 18."