@Ian they are in a legally valid relationship once a person reaches the age of consent, which ranges from 14 to 21, depending on the country. In Taiwan, where they both live, the legal age of consent is 16. A consensual relationship between a 17-year-old and a 27-year-old stranger (I say “stranger” because the ML is not a teacher or guardian to the MC, which would change things) is generally legal, even if many people might still find the age gap morally uncomfortable.