While years pass within the story, Rin remains a young child throughout the entire series. This is likely due to:
* Her connection to the spirit world: Rin's connection to the spiritual realm might affect her aging process.
* The story's focus: The manga primarily focuses on InuYasha and Kagome's journey, and Rin's physical growth is not a major element of the narrative.
It's important to remember that the manga is a work of fiction, and the creators can choose to depict characters' ages however they wish.