The point is that when you evaluate the success, and the degree of success, of "aspects of Japanese lifestyle" that promote longevity, you need an accurate assessment of how long-lived Japanese people are.

We get a very different understanding of the value of, say, the Japanese diet if Japanese life expectancy at age 2 is 84 versus if it's substantially lower.

Do you actually meet the grandparents, or do you just hear stories?

Hearing something and saying it is hearsay.