Thinner, younger females are more attractive

Scientists say that fit females of a certain BMI are most sexually appealing. The study was funded by the National Science Foundation of China and involved researchers from 10 different institutions around the world.

Professor John Speakman explains:

“Fitness in evolutionary terms comprises two things: survival and the ability to reproduce. What we wanted to investigate was the idea that when we look at someone and think they are physically attractive, are we actually making that assessment based on a hard-wired evolutionary understanding of their potential for future survival and reproductive ability?”
Age is itself a strong indicator of evolutionary fitness. When the age factor was included into the model the optimum fatness fell to a BMI somewhere between 17 and 20 – corresponding exactly to the images people found to be most attractive.
This suggests that we find thinness in females so attractive because we equate it with youth – a BMI of 17-20 corresponds to the average BMI of a young 18-20 year old with maximal fertility and minimal risk of future disease.

Here is a link to the original article: https://www.abdn.ac.uk/news/8098/

Here is the problem I have with this study. I could not find any data indicating that the average BMI of 18-20 year olds fell within the optimal BMI range of 17-20. Wiki shows that females age 10-15 on average fell within the optimal BMI range.
https://en.wikipedia.org/wiki/Body_mass_index

Here’s the wiki data graph from taken from the CDC.