How is Metamorphosis by Franz Kafka a coming of age story?

Like with most of the work of Franz Kafka, "The Metamorphosis" can be interpreted in innumerable ways. Among other things, it can quite justifiably be read as a coming-of-age tale. In the novella, Gregor Samsa still inhabits a room in his parents' home when he is suddenly (and inexplicably) transformed into a giant insect. As his family recoils in horror at Gregor's new form, he is ultimately outcasted from his own home, and, once he dies, he is completely forgotten about.
In a sense, one can read this tale as a metaphor for the transition into adulthood. At a certain point, children become adults and are forced to leave the homes they grew up in. While not entirely forgotten, they are soon removed from the daily routines that occur in the house; Gregor is literally treated as a sort of grotesque inconvenience by his family. They hope for him to leave because he is no longer wanted.
Meanwhile, Grete, Gregor's sister, becomes increasingly dissatisfied with taking care of him. She, too, begins to grow older as she acquires her own responsibilities and duties. Gregor becomes an increasing burden on her and the rest of the family. While Gregor initially rejects this transformation and the stresses it puts upon him and his family, he soon dies; his family is relieved. Gregor's transition into an independent adulthood is ultimately a failure. He is a burden and is unable to live alone.