"Soon to be"?, already there:
"The DSM-5 requires that a person must be at least 16 years old, and at least five years older than the prepubescent child or children they are aroused by, for the attraction to be diagnosed as pedophilic disorder"
From the first chapter:
"I'm Kohinata Riko, and I'm a 2nd year."
If you are on the 2nd year of high school then that means you are already 16 years old on the first day of the school year. So yeah, Riko is already 16 years old since the first chapter.
Also, the new laws about age of consent approved not so long ago in Japan:
"In 2023, the two articles were amended to raise the age of consent from 13 to 16 years old. An exception was included for sexual acts involving persons aged 13 to 15, by which the older partner is only subject to penalty if five or more years older. The amendments were part of a larger reform of Japan's sex crime legislation."
Seeing the references to 16 years old and less than 5 years of difference, it seems they were literally taking the DSM-5 as reference when crafting that law with only a slight tweaking of the lower limit.