As we were talking about the sexes, I thought you meant he was discriminated against just for being a male.
Now that you mention age, this starts making more sense now.
Age discrimination is real, especially towards foreigners because unlike Japanese citizens, foreigners can just flee unexpectedly and foreign courts will not rule in the landlord's favor while national courts will be like "he has no residence in Japan and no Japanese nationality, so we can't take your case sir".
Whether him being single or married has nothing to do with it really.
Otherwise you'd be denying the fact there's an abundance of single men renting apartments and mansions without any problem whatsoever nationwide.