Traditional markets in Jongno-gu still use Korean age counting. You're born at 1 year old, everyone ages up January 1st regardless of birthday. So your international 20 becomes Korean 22 when buying alcohol at Gwangjang Market vendors.

Don't argue with the dried fish ajumma about your age - she's counting Korean style and fighting it just marks you as clueless foreigner. Accept your temporary promotion to elder status and move on.