According to MY RESEARCH, that I've done extensively on. InuYasha shouldn't be 200+ years old. And here's why:
I’m looking into the fashion and the timeline of InuYasha. Mainly the 12 layered robe that Inu’s mother wore. We all know it’s a Heian Period garment worn by noble women and ladies-in-waiting at the Japanese Imperial Court.
Here’s an interesting fact: “Despite it’s name the jūnihitoe varied in its exact number of layers. The number of layers, and the type of layers, could alter the formality of a jūnihitoe outfit, with some accessories, such as overcoats and a long, skirtlike train (known as the mo) only worn for special, formal occasions.”
The Heian Period was from 794-1185. And InuYasha takes place within the Sengoku Jidai or the Warring States Period. This was sectioned off within the Muromachi or the Ashikaga Period which was from 1333-1573.
Going back to the InuYasha’s mother, SHE CAN’T be of the Heian Period. Yes her hair was loose, down her back and pretty long this seemed to be the style going until the Muromachi/Ashikaga period which our story takes place in. So from 794-1336 the hair style for women of noble, royal and aristocratic birth (even those who had fallen on hard times and were in poverty) was to keep it long and free. And TRUST ME guys I LOOKED FOR HAIRSTYLES FOR THE KAMAKURA ERA. I looked and I found nothing. The only thing I found was the long hair for the women in the Heian Period and then it went to the ponytail thing we see in the series proper though I’m sure that the ponytail thing came in gradually so it the long hair could have stayed in style for awhile in the Muromachi since hairstyles don’t usually change over night. That takes a while for it to happen.
The Sengoku Jidai was from 1467-1615.
And the original series takes place in 1546. Especially since the introduction of firearms into Japan happening in 1543. And the Band of Seven were executed 15 years before the start of the series, it was stated as such, so they’d would have been executed in 1528. And Kagome meets that one guy from the Oda clan who tells her he’s not Oda Nobunaga who at this point is still referred to as the Idiot/Big Fool of Owari. He was born in 1534. And with InuYasha being bound to the Tree of Ages for 50 years. We’re looking at 1496 for his sealing to the tree.
Why 1496? Because 1996 is the modern era in which Kagome lives in due to her fashion. She’s wearing late 90s fashion. 1996-1997 school year places her in the 3rd year of Jr. High (which for us in the US would be 9th grade high school freshman) since she has to worry about high school entrance exams in that school year. 450-1996 = 1546 when Kagome goes 450 years back into the past. And it CAN’T BE 500 years because that’s when the sealing takes place in 1496. She arrives 50 years AFTER the sealing.
1496+50 = 1546.
1546+450 = 1996
It’s still Kagome’s birthday 450 years in the past. She had JUST turned fifteen so that makes her birthday c. Early April 1981 to be in the 1996-1997 school year in the modern era.
March 1547 is when Naraku and the Shikon Jewel is defeated and taken out of existence.
3 year time skip puts the year Kagome graduates high school in mid-March of 2000. She’s 18 by this point. And the Since Then special chapter: 6 months.
1547+3 = 1550
Kagome returns in mid-March of 1550 making her 18. InuYasha Since Then happens 6 months later in mid-September of 1550. She is 19 six months later.
Kin'u and Gyokuto are born a year later after Naraku’s defeat in 1548 with Hisui being born just after Kagome come back in 1550.
The anime is the only one to give specific dates (200 years before, etc, etc) the manga DOES NOT. I’ve read the chapters that deal with Ryūkotsusei. In the anime it says 200 years BEFORE the shattering of the Shikon no Tama. In the manga there is NO DATE GIVEN. So by nature the anime contradicts the manga and the manga holds up as that is the source material and the anime can’t go against the manga in that way. The studio, Sunrise, is the one that made the 200 years before the shattering of the Shikon no Tama a thing.
The manga said that InuYasha’s dad died from wounds that were in result of his battle with Ryūkotsusei. The 3rd movie states when this was and how he died, but it again was a product of Sunrise. Rumiko did some character designs and some storyboard treatments I’m sure, but everything else from a timeline perspective was solely the studio’s decision. The films are side stories and are not canon to the timeline of the series.
So in terms of timeline InuYasha stopped aging in the human sense once he hit puberty and he looks to be about 15 years old. And puberty for HUMAN BOYS is between 12-16. And if he ages until he hits puberty then he hit it at between 12-15 years old in humans years and stopped aging like a human once that happened. He states both in the third chapter of the manga and the 2nd episode of the anime that he doesn’t have to worry about getting old, at least for a while. INUYASHA is 150 years old IN THE ORIGINAL STORY. His birth year has to be around 1396 for the sealing to take place in 1496.
Birth Year/the latest possible year that Inu’s dad’s death: c. 1396
Inuyasha hits puberty and stops aging at a human ate: c. 1409-1411
Sealing to the Tree of Ages: 1496.
Unsealing from the Tree/Kagome comes to the Sengoku Jidai: Early April 1546.
Defeat of Naraku: March of 1547.
We DON’T KNOW WHEN EXACTLY OR HOW INUYASHA’S MOTHER DIED. Just that she died a long time ago. So it’s hard to pin down exactly. But for sure she wasn’t born and lived in the Heian Period and Inuyasha isn't 200+ years old YET.