The indecisiveness and inaction with which Emilio deals with affairs in his life lead him to shut out his memories, leaving him in a state of spiritual old age (hence the title "Senility").

At the start of the novel Emilio meets Angiolina, a vulgar, poor but beautiful woman, and falls in love with her, causing him to neglect his sister and his sculptor friend Stefano Balli.

Emilio, jealous of Balli, becomes progressively estranged from his sculptor friend, and Amalia, knowing that her secret love is hopeless, numbs herself with ether.

She ultimately becomes ill with pneumonia (Amalia's attitude to death is similar to the suicide of Alfonso Nitti, protagonist of Svevo's first novel Una Vita, "A Life".)

The novel ends with a meaningful but poignant image: years later, in memory of this special moment of his life, Emilio begins to merge these two women into a single person – an idealized woman with the beauty of his mistress Angiolina but with the pious heart of his dead sister Amalia.

Greatly appreciated by James Joyce, who strove for the publication of the second edition, the novel approaches the style of the Irish writer in that it is essentially introspective and aims to shed light on the inner life of the protagonist Emilio.

It is now considered a classic of Italian literature, with some critics like Joyce, Valery Larbaud and Eugenio Montale, admiring it even more than his later and more famous novel, La coscienza di Zeno.