canon wise, the manga in "modern" time starts in 1997 kagome just turned 15, meaning that her birth happens in 1982.
Inuyasha's time is said to be around 500 years in the past, if you read the manga/watch the shows the only REAL time line you'll get is by looking at the clue's that Rumiko left.
IE: "the woman that loved Sesshoamru" this is NOT canon btw, but it was in the anime, he's shot by muskets, and they were not introduced to Japan until 1543
en.wikipedia.org/wiki/Firearms_of_Japan
Other things are Nobonaga, who steals kagome's bra called Oda a fool for trying to unite Japan, something that happened in 1561 - 1582 when he died, and his heir finished his job.
there are other time line clues, but most of them seem to point that Inuaysha the anime/Manga happened somewhere around 1540ish - 1580 ish, which is really only 425 years givertake, not 500.
Non-canon wise, if you look to movie number 3, Izayoi is a hime that often is seen wearing many layers of kimono, a J?nihitoe,
en.wikipedia.org/wiki/J%C5%ABnihitoe something was was started in the 10th century, and declined used happened in the 14th century. Meaning if Izayoi was still wearing them Inuyasha had to have been born sometime around 1300, making him somewhere between 150 - 200 years old when the manga starts off with Kagome. this would also made Sesshomaru much older as he already looked "17" when his half-brother was born. and has "aged" 2" years while 150 - 200 have passed.
I hope this all helps!
- r0o
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