Also, when you use these words on a human being, it can also be used to refer physical and mental maturity. To say someone is 젊다 emphasizes that they are physically young and healthy. To say someone is 어리다 can emphasize that they are mentally young and not quite mature yet.

젊다 means "to be close to the age of adolescence(late teens)" 어리다 means "to have fewer years old" 젊다 is used only for adult human. 어리다 is used for human, animal or plant. Thus, 젊다 is commonly used for a person older than late teens 어리다 is commonly used for a person younger than late teens, animal, plant, etc. a. 제 동생은 당신 동생보다 2살 어려요. b. 그는 저보다 어려요(younger) / 그는 저보다 젊어요(only for adult). c. 할아버지가 할머니보다 젊어요. (only for adult). d. 제 언니가 어려 보여요.(younger)/ 제 언니가 젊어 보여요.(younger and closer to late teens) e. 그녀는 머리를 짧게 자르면 어려 보여요(younger)/ 그녀는 머리를 짧게 자르면 젊어 보여요. (younger and closer to late teens)

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