Japan has been known as a nation with exceptionally long life expectancy. Now a slightly embarrassing partial explanation has been found: the exceptionally old are often not actually alive. Many 100-year-olds are missing. The person who should have been Tokyo's oldest male resident at 111 years, Sogen Kato, had been dead for about 32 years. The "oldest woman" - or so thought to be - hasn't been seen by her daughter in 20 years.
There is concern that the loss of connection between the very old and their offsprings is because of loosening family ties, traditionally important in Japan. However, there is a simpler explanation: while Sogen Kato's body was mummified in his apartment, his pension payments kept coming, and they went to the family. Strict privacy laws helped this to go on for decades.
Perhaps the inverse age pyramid problem in Japan can be relieved simply by requiring old people to show up every now and then.
Note: this can't happen in Finland. We actually can do a very reliable census without hiring lots of officials to knock on every door. Here it is enough to say something like SELECT COUNT(personid) FROM vrk WHERE alive = TRUE; and there you have it. A few people of selected minorities might cause minor inaccuracies, but it'll be insignificant.