second childhood
noun [ S ] uk/ˌsek.ənd ˈtʃaɪld.hʊd/ us/ˌsek.ənd ˈtʃaɪld.hʊd/
（尤因年老智力衰退造成的）第二童年期
a situation in which someone starts to behave like a child, especially because of mental weakness caused by old age
Her grandfather's in his second childhood and talks nonsense most of the time. 她祖父年老昏聩，大多数时候总是不知所云。