Half the most severe cases in France are under 60 years of age. It begs the question of why Japan is also not seeing anything like that.
Deaths are certainly one thing to consider in terms of age, however if Japan had a mass spread of the virus I would expect to see a lot of ICU cases regardless. Even when countries aren't testing properly, the intensive care cases (or lack thereof) reveal what's going on eventually. I think it goes beyond the isolation of old people for Japan.
It's similarly puzzling as the German case to death ratio, versus eg Spain (comparable case counts, 15 times the deaths). I've seen that one explained that older people in Germany are far less likely to live with their families vs Spain or Italy (so fewer of the most vulnerable are catching it in Germany). And yet, Germany also has three times the cases of Switzerland, with the same number of deaths. Something is very strange there. The low recovery figures out of Germany might imply they're not deep enough into the outbreak, time wise, to have the death rate spiking yet.