Early centenarians

Have a look at how many people from each of the nations you mention were killed in both world wars. This page says itself that it’s not definitive, and offers links to other lists, but there’s a big difference between 1.6 million dead (Germany, WWI) and 300 dead (Japan, WWI), or indeed 9.5% of your population (Germany, WWII) and 0.4% (USA, WWII). Who knows how long these people would have lived if they hadn’t died, so to speak?

It would be my guess that it has to do with the strict privacy laws in Germany. As far as I know, it is practically impossible for a third party to gain access to birth certificates, death certificates etc., sometimes even concerning persons who have long passed away.

As a consequence, this type of information is not readily available for researchers.

True, but you have to also keep in mind that men who were born in 1901 or later were never called up for service. And in addition to that, many of the German men who were born in the early 1900s were too old (or unfit) to serve in WW II (many others were, however, drafted near the end of WW II)

Worth bearing in mind, but also worth bearing in mind is that huge proportions of the German deaths in particular were civilian - without looking back at that page I linked to, I think more than half the deaths in WWII were not military. Whereas every American death was military and would therefore be covered by your caveat, age is no barrier to being killed as a civilian.