The present specification relates to a composition for improving cognitive decline including a novel compound isolated from post-fermented tea, its isomer, its pharmaceutically acceptable salt, its hydrate, or its solvate. Yes, it can be widely used in the fields related to cognitive function and nerve cell protection. [Selection] Figure 6本明細書は、後発酵茶から分離した新規な化合物、その異性体、その薬学的に許容可能な塩、その水和物、又はその溶媒和物を含む認知機能低下改善用組成物に関するものであり、認知機能及び神経細胞保護関連分野において広く活用され得る。【選択図】図6