Japan's age gap isn't due to them having absurdly good healthcare. It is due to them having a population explosion through the 1900s-40s, which played a large role in them trying to create an empire (more land to dump the excess population off on). That tapered off a bit after the war with a fertility rate of only around 3 children per woman throughout the rest of the 40s and 50s. Starting in the 60s it fell to 2 and they have been below replacement levels since the 70s. They also have almost non-existent immigration so they have no real way of bolstering the much smaller younger generations.