"- Last but probably very relevant: they have a completely different place of the elders in society. In Japan, the elders often live alone, sustain their life by themselves: it's a matter of proud. Even contact with their own kids is often sparse. Meaning, probably also lower risk of infection. "

The elderly in Japan are extremely active and are very well socialized with other elderly thanks to public transportation and compact cities. They are very independent and mobile. Considering that Japan never took active quarantine measures on a nationwide scale, it's honestly a mystery why Japan is an outlier.