Soy nativo español, y ahorita estoy viendo Inuyasha con el doblaje en español y tengo que decir que suena genial y a veces juegan con el doblaje, hay una escena donde Kagome (Aome en el doblaje en español) canta "mesa que más aplauda", Shippo dice "¡chiquito pero picosito!" y Miroku (Hachiemon como Miroku) también dice "¡fuera de aquí, espitirus del mal!" (¡ojo! "espitirus" está mal pronunciado, debería ser "espíritus", ¡pero eso es lo que hace divertido el doblaje en español!) More on reddit.com

If you recall, there's an episode (143 I think) where Shippo meets a little otter demon named Kanta. He has a flashback during the episode to his own father, who is as large as Kanta's father, while Shippo and Kanta are very small. I'd say the best answer would be that they age very slowly. But we don't know exactly how old they are. I don't think he's eternally in the form of a child either, again, demons grow slowly so he will get larger in time. Remember, even as a half demon Inuyasha took around 150 years (not counting the 50 years he was sealed to the Tree of Ages) to get from little boy to young adult. More on reddit.com

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He's a (fox) demon they age much much slower More on reddit.com