In Japan, 20 is the age of turning into an adult, both traditionally and legally in many cases. There’s a “coming of age day” - 成人の日 - which is for celebrating those who have turned 20 in the last year. Many legal provisions relate to being 20. The voting age used to be 20 until very recently where it got lowered to 18, and I believe drinking is still only allowed at 20. Probably other things, too.
The age of consent varies by prefecture, but I believe the latest age is 18. It’s not tied to the above “becoming an adult”.
So basically - society still views Nakajima as a kid because she’s not 20 yet. But sexual-consent wise they’d be legally okay to get dirty. Which is good because, y’know, their current situation would be grooming otherwise.