It's really not that uncommon. IIRC the main couple in Love Sick's actors were 17 & 20 when it first began airing.
How is their mental health at stake? They are literally just playing the role they were given? It's not a big deal? They and their parents consented to it.
Links here - https://kisskh.at/discussions/the-first-love-story/65675-where-to-watch