Beloved is indeed a "coming-of-age" story. While it may be thought that the "coming-of-age" aspects of the story revolves around Sethe, the aspects of "coming-of-age" revolves around Sethe's daughter - Denver. By using the various events of the story that affect Sethe, Morrison is able to create a situation where Denver is forced into leaving her home and entering the world as a woman instead of a child.
At the beginning of Beloved the character of Denver is portrayed as little more than a child. Denver's hunger for company, seen through her desire to play with the ghost of 124, allow the reader an early grasp at her situation - while Denver is eighteen years of age she still acts as though she was half of that number. Every moment of Denver's life is also spent within the confines of 124, as is seen throughout the novel, the few times that Denver has left 124 occurred early in her life and after the events which drove her brothers away from 124.
Denver's first actions towards Beloved also shows her more childlike personality. Where an adult would treat Beloved (a stranger...) as a normal human being, Denver chose to treat Beloved to the best that she could and desired every second of Beloved's time - such as a child seeking attention or in front of an idol of theirs would act. From what can be seen, Denver's personality does not change from her early childhood to her late teens; she remains a static character until she is forced to finally reach outside of the shell she has built up around herself.
Where the story changes from a static one (concerning Denver) into a "coming-of-age" story is when Beloved and Sethe "face off" with each...