At the medicine contest (where MC represented the black market) she was introduced officially to "uncle" (Lord Hell in disguise, him being teacher of an (elite?) academy), and told uncle she was 16 years (shocked elder/master Ke (younger brother)) and him being 25 years.
I think only half a year has passed since. A 1-2 months as prisoner of Lord Hell (here MC also cured her scars) + ~1 month escaping and returning home + 2-3 being home handling business (THOT begone) with minor travels.
16-17 years old is being an adult. Consider the setting, no point comparing to real life.