I am writing this under the premise that you meant the poem "Old Man Moon" by Aileen Fisher. It is translated "달님은 노인" in Korean, because there is no apposition("동격/同挌") in two nouns in Korean grammar. In Korean, if you use two nouns in paralllel, it becomes a complex noun. By using the apposition, you can allocate two nouns in English, such as "I, Robot," which means "I am a robot." By contrast, in Korean, if you allocate two nouns, it becomes a complex noun, which means that the first noun is generally understood as the modifier of the second noun. Here, in the original text in English, the two nouns are used in apposition, which means in a metaphor that the moon is an old man (because it ages in a month). However, in Korean, by allocating just two nouns as "달님 노인"(moon, old man), it becomes a complex noun (and the first noun is the modifier of the second noun), which means a specific old man who is somehow relevant to the moon, and that is not the same with the meaning of the original text. Therefore, the translation such as 달님"은" 노인 is closer to the meaning of the original text in English due to a grammatical reason. Hope this helps! :)

Can you tell me more about the context? It will help to answer your question, I guess! :)