RetireJapan

What would be a sweet spot in the age of construction of mansions or houses for investment purposes. Not too new so it has already depreciated, and not too old that it may become worthless soon. I see a lot of 1990 era mansions for sale in my budget, but not sure if I should go a bit newer.

I researched that a lot before buying my mansion this month.
Here is a paper from Nomura with tangible data:

Obstacles to affluence: thoughts on Japanese Housing
https://www.scribd.com/document/237374520/Np-20081I37

It is from December 2008 but still better than mere opinions.

However you do not want to buy something that is not Shintaishin (latest earthquake resistance norm from April 1981).
The risk is very real:

Look at the numbers from 2011 Tohoku Earthquake
https://r-sendai.com/blog/renovation03-2/

And from 2016 Kumamoto Earthquake
https://locust.co.jp/contents/66

I also heard there are four varieties of construction materials for mansions and they are quite distinct in terms of re-sale values.

Forgot the names, but they relate to things such as steel-reinforced concrete, steel framed and panel type, some wood, etc.,

They are also likely to be quite mixed in the generation of apartments, mansions etc. you might be looking at if you are looking for something that has depreciated 70%, which I’m told be the way doesn’t mean linearly depreciate as they flatten out toward the bottom, which makes older good as long, as you say, you get passed the earthquake test standards.

We bought our manshon a few years ago: it was 26 years old at the time (which meant we couldn't get the mortgage interest tax credit) but we also got it for about 1/3 of the price it was when it was built.