Yeah they're living in an era where most girls, especially noble girls, would be married before they're 15 and have children basically as soon as they're fully grown, aka around 15~16. Author is being silly.
Also, Koreans measure age differently than the rest of the world. A 19 year old Korean is 18 years old to the rest of the world, because they consider a newborn baby to be "one year old". Not only that, but Koreans don't celebrate normal birthdays. Instead, everyone gets one year older on new years. So if you were born on December 31st, then you'd be "two years old" only one day after being born.