People leave a pub during curfew hours in Seoul earlier this month. (Kim Hong-Ji/Reuters)

SEOUL — For a cohort of teenagers on the cusp of adulthood every year in South Korea, the night of New Year’s Eve is one big birthday party.

Age in South Korea is traditionally calculated by birth year, not the exact birth date, so at the stroke of midnight on Jan. 1, the 51-plus million people living here effectively become one year older. For those turning 20 in Korean years, the legal drinking age, midnight is when the door opens to nights of lawfully clubbing and downing shots of soju.