Lol stop the devil's advocating.
"Teen" in its plainest definition just refers to the numbers 13-19. "Adolescence", the concept of there being a period between childhood and adulthood, didn't enter society until the 1920s, I believe, when there was a big controversy about little girls getting raped in Europe. People knew that shit needed to stop, so the age of consent was widely raised to 16 in most places.
"It's just a social construct" is a dumb argument. Men in their 30s going after teenagers are looked down upon for a reason. No.18 calls Kurilin チビのオッサン (chibi no ossan), which basically means "small-sized uncle"; オッサン (ossan) is short for おじさん (ojisan), which literally means "uncle", and is used as slang for an older guy. She didn't see Kurilin as being of the same generation as her at all, despite the fact they were both technically adults.
Obviously, there's nothing legally wrong with 18-19 year old girls fucking and marrying men in their 30s, but it's looked down upon. More often than not, the older guy doesn't have pure intentions. Sure, Kurilin's a submissive beta cuck, but it's still cringe lol.