Nobody uses the term generation gap anymore, but the problems it refers to are eternal—and they’re the core of When You Finish Saving the World, Jesse Eisenberg’s debut as a writer-director. Finn Wolfhard is Ziggy Katz, a teenage singer-songwriter who’s a mini-sensation in his corner of the internet, cultivating an audience of loyal followers with his folky songs about youthful love and angst. But his mother Evelyn (Julianne Moore), the harried founder and manager of a women’s shelter, downgrades both his enterprise and his talents, and his father Roger (Jay O. Sanders) is a checked-out academic type whose nose is always buried in a book or a magazine. The clash between mother and son is particularly acute: Evelyn just wishes her son were some…