One thing I noticed very early on is that most writers reffer to Inuyasha's or Sesshoumaru's clothes as "haori". Let me set the record straight here: they are not wearing haori! A haori is an overcoat, with or without sleeves, worn over a kimono and hakama outfit; it's open at the front, except for a cord keeping it in place. If you have watched the episode "Only you, Sango", the lord that asks Sango to marry him wears a haori. You could say it's formalwear.
Inuyasha is wearing a yukata (cotton kimono, usually off-white when used as undergarment or sleeping attire), a hakama (pleated pants), and a gi (shirt, or waist-length kimono if you will). Sesshoumaru, on the other hand, wears a kimono (ankle-length garment, usually silk), a hakama (worn over the kimono), as well as an obi (sash).
There are a few details I would like to point out about Sesshoumaru's clothing as it relates to his age. If you notice his hakama, it is rounded at the bottom. Moreover, the sleeves on his kimono are wide (hirasode or hirosode), while his obi is tied at the front (they were later tied at the back). In addition, he wears actual boot-like shoes instead of geta (wooden sandals) or zori (sandals made of rope). This particular style was popular during the Heian era (794-1185 CE). Considering Kagome has gone back about 500 years to the past, then the story must take place during the late Muromachi period (1392-1573 CE) . But enough with the history class. What I'm trying to get to here is that taking into account these details, it would be safe to assume that Sesshoumaru is at least 400 years old!
I know what you're going to say: Inuyasha's hakama is also rounded at the bottom, but he's not that old. Well, we know that Inuyasha was 150 years old when he got sealed to the Goshimboku (yes, that is the correct way to spell it!). Even though 50 years have passed since then, he still acts like a teenager - you could say 10 youkai years amount to 1 ningen year. But we also know that his fire-rat clothes were given to him by his father. So, logically, Inu no Taisho was around during the Heian era (and had the outfit made during that time), and considering he's a taiyoukai, he may have been even older than that.
One last thing I'd like to mention concerns housing. Japanese-style houses do not have doors; they have shouji (sliding doors made of paper) - and these are not sound-proof, people. In addition, houses are built slightly above the ground to avoid damages due to flooding. The floors are wooden, but tatami (straw mats) are laid in all the rooms and corridors inside the house. The rooms have several uses; there are no bedrooms strictly speaking, for instance. You would sleep on the tatami floor on futon (mattresses) that would be stored away in the morning. Finally, a note about offices: there are no rooms for that particular use (except for large rooms, where a lord would meet with his advisors), and no desks other than small, low tables used to write on (kinda like laptops!) while sitting on the floor. As for books, they are not bound, but in paper rolls (very practical to write on BTW when using vertical right-to-left script, provided you're right-handed of course). The later are stored in hive-like shelves.
Well, that's it for now. I hope this can help all the great writers out there make their stories more accurate and plausible. Jaa ne ;)