In Japan, the national age of consent is 16, and prefectural laws are intentionally written vaguely to give prosecutors leeway in dealing with Japan's sex industry. They usually only say that they ban 'indecent acts' with an adult. However, that isn't taken to mean sex in general and is typically prostitution or assisting minors in circumventing age-based restrictions around other things.
But Takamura and Senko live and work together, and their relationship is acknowledged by her parents, so by Japanese norms, it would be extremely strange for it to be treated as a crime.
This is why you will often see manga where someone in their early 20's and a high school student start dating have a scene where they get parental approval, because that is an actual normal thing that happens in real life for these exact reasons.
