So 상투 is a married men hairstyle. In old times, marital status was very important. If a child was married, he's treated as an adult with formal speech, and an old unmarried man was treated with informal speech. In Joseon dynasty, early marriage for men was common. (cont.) https://twitter.com/CheriKookie/status/1247234914220740611

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Sometimes unmarried men had 건상투 to pretend he's married so ppl wouldn't look down on him. But it only worked outside of his village since his neighbors knew he's not married So unmarried men could have 상투, but for drama, it might be intended error for love line

Also, historical dramas usually have many research errors so don't take everything as historical resource! It's a good start to get familiar with Korean history though! Enjoy your drama