Inu-Yasha, on top of being sealed for 50 years - being half demon has always aged differently than that of regular humans.
In June 2001 Rumiko Takahashi talked about how Inu-Yasha ages:
Source - https://websunday.net/5657/
"犬夜叉は半妖ということで、どーゆー歳のとり方をするのか謎な上に50年封印されてたので難しいですが、50年はフリーズしてた事として…。心情的には、人間に換算して、かごめと同じ15歳くらいが良いのではないかと思ってます。
七宝、殺生丸、鋼牙などの妖怪は、何十年何百年単位で生きてそうなので、やはり読者の皆様の見た目で判断していただければと思ってます。"
"Since Inu-Yasha is that of a half-demon, the way he ages is a mystery, and he was sealed up for 50 years, it can be difficult to say, but let's assume that he was frozen for 50 years…. Emotionally, I think it would be better to be about 15 years old, the same age as Kagome, when converted into a human being.
As for demons like Shippou, Sesshoumaru, and Kouga, they can live for dozens or hundreds of years, so I think it would be best for the readers to make their own judgements based on appearances."
Following up this statement - In 2003 the InuYasha Profiles Book was released in Japan.
The characters listed above in the past interview on websunday in the Profiles book as follows:
Shippou
Age: Seven (in human years)
Sesshoumaru
Age: Nineteen (in human years)
Kouga
Age: Fifteen (in human years)
InuYasha
Age: Fifteen (in human years)
"In human years" is only specified for demon race characters.
This is confirmation that these characters are older than how they appear.
Within Chapter 3 of the Manga and Episode 2 of the anime both mention aging -
We have also seen young Half-Demon Inu-Yasha and Shiori within the series. With both their mothers.
On December 20, 2003 InuYasha the third movie released and within that we learned that Inu-Yasha's father - The Inu no Taishou died 200 years ago, the night he died - Inu-Yasha was born. Confirming that Inu-Yasha is in fact hundreds of years old.
In 2013-2015 the InuYasha Manga were re-released as definitive Wide Editions that included Explanations and Interviews written by Rumiko Takahashi
In InuYasha Wide Edition #1: https://www.tumblr.com/officialinuyasha/658587431804846080/inuyasha-wide-edition-1-translation-inu-yasha
"-BODY- Strong and enduring
Although Inuyasha was sealed in a tree for 50 years, his body never grew old."
"he grew up constantly fighting"
Rumiko Takahashi also mentioned the third InuYasha movie in InuYasha Wide Edition #17
https://www.tumblr.com/officialinuyasha/665797128914944000/inuyasha-wide-edition-17-translation-inuyashas
"The story of InuYasha’s father revolves around the fact that InuYasha is a half demon. His mother had to be a human and I wanted his father to be a very powerful demon. That’s why I created him as a great demon who ruled over the Western Lands. At the end I only drew him alive once, in his dog demon form, but I never showed his human appearance. Anyway, I’m satisfied because in the animated movie “Swords of an Honorable Ruler” (“Tenkahado no Ken”) I could see him alive with his human appearance. I also saw InuYasha’s mother alive and I liked that."
We know that Rumiko Takahashi had ideas that she couldn't fit into the manga, so these were used in the anime and movies. This was mentioned at the InuYasha Panel at Anime Boston 2018 -
https://www.youtube.com/watch?v=acn1q9X30qs
https://www.tumblr.com/officialinuyasha/678254881205895169/inuyasha-panel-anime-boston-2018
Michihiko Suwa: "In the actual animation, she didn't have much to do with the production. But we did have her review all of the screenplays (scripts), some of which were written by Mr. Sumizawa next to me. And, She also looked over all the storyboards and she'd give us feedback - and you know she always reserves - the creator always reserves the right to refuse or ask for changes. But, in our case, the team was so brilliant that - for the most part she approved everything without much criticism. So, I might have Mr. Sumizawa say a word or two about his interaction."
Katsuyuki: "I have to say, among the many creators I've met and worked with, Takahashi-sensei is a particularly a very sweet individual. She's very kind, and I often go to her especially when we were writing side stories and I'd say "I have this idea for this episode!" and she would actually show genuine interest. And so it was a very easy project for me to work on. I don't know if we are going to be talking about the films later - But I actually also did the screenplays for all four of the films. The feature films, and sensei was really generous and saying "You know, there's this stuff I want to cover in the manga, but I can't get to it. What do you think about this - What do you think about that?" and all I had to do was then flesh it out into an actual dialogue and background. So, it almost seemed like a very synergistic chemical reaction, and of course it wasn't just the two of us - you had the directors, the animators and so forth. But, I have so much fond memories of working on InuYasha - That I'd be excited if we could work on yet another, new InuYasha project. Would you like to see one too?"
It was also mentioned in Animage October 2020 that Rumikos ideas were used in the non-manga stories and they agreed on a number of settings exclusive for the anime version.
https://www.tumblr.com/officialinuyasha/629334377576464384/this-was-a-translation-collab-between-esther-xiao
Sumizawa: "In the movie "Inuyasha: Affections Touching Across Time", the setting "The sacred tree was the material used to make bone-eater’s well" was an idea given by Rumiko-sensei. Readers and fans may not have known but even for the non-original parts of "Inuyasha", while discussing with Rumiko-sensei many times, we decided on a number of basic settings unique to the anime version and that’s how we made the work."
Rumiko also refers to the anime scripts as official answers!
In Shonen Sunday Super December 2020 -
https://ayuuria.tumblr.com/post/633091321063784449/inuyasha-yashahime-translation-shounen-sunday
Q: I want to know how Sesshomaru and Jaken first met. Was Jaken an old man from the time they first met?
I remember their meeting being drawn in an original episode in the “Inuyasha” anime. I can’t remember which episode number though. According to that, Jaken was a demon leader who lost to Sesshomaru in battle and thus decided to follow him, I think that was the scenario. And I think he was an old man as well.
We also see adult Shiori in the Yashahime: Princess Half Demon Anime!
On Takashi Shiina's Blog 07/27/2023
https://cnanews.asablo.jp/blog/2023/07/27/9605339
Takashi Shiina: "アニメでは半妖の孤児たちを保護する活動をしてましたが、もしお母さんがいまも元気で彼女を勇気づけてて、もし人間社会と折り合いのつく生き方を見つけていたとしたら・・・海辺の村にいたキャラクターだったので、成長後は船で退治屋をしているというのはありそうな話に思えます。"
"In the anime, she was working to protect half-demon orphans, but if her mother was still alive and well and encouraging her, and if she had found a way to live in harmony with human society…she was a character from a seaside village, so it seemed like a possible story that she would grow up to be an exterminator on a boat."
In Shounen Sunday Super December 2020
https://ayuuria.tumblr.com/post/633091321063784449/inuyasha-yashahime-translation-shounen-sunday
"Q: In “Hanyou no Yashahime”, Inuyasha and Kagome’s child, Moroha, appears. Inuyasha is a half demon, Kagome is a human, and Moroha is cross between a human and a half-demon. Do the three of them age differently in the Feudal Era?
Rumiko: "In the span of the story, they seem to age the same. I’m sure the results will gradually begin to show after a few decades."
We know that InuYasha would still look young, like Sesshoumaru after a few decades.
In InuYasha Wide Edition 4, InuYasha is said to have taken place during the 1540s
https://www.tumblr.com/officialinuyasha/664010799055044608/inuyasha-wide-edition-4-translation-sengoku
Yashahime: Princess Half-Demon - Episode 21 - "Secret of the Rainbow Pearls" Summary from Animedia March 2021
https://www.tumblr.com/ayuuria/642660474027196416/yashahime-translation-titles-and-summaries-for
Added weight to this. It states that Taishou died in the Kamakura Period, which takes place during the years of 1185-1333.
You can see our thread on how we calculated Inu-Yasha's birthday year here - https://www.tumblr.com/officialkagome/172468163585/when-is-inuyashas-birthday-ive-been-seeing
It's direct confirmation that half demons age physically up until a certain point. At about puberty, then stay young-looking for a very long time even if they're physically hundreds of years old.
