The Beatles song "When I'm 64" specified that age as one of retirement and grandchildren. Today, the mid-60s seems a tad too young for both those things. But consider this: Back in the 1960s when Paul McCartney wrote that song, the average life expectancy worldwide was 52 years old, according to World Bank data. Fifty-two!
Fast-forward to present-day Japan and 52 is まだまだ若い (mada-mada wakai, still quite young). Some people marry at that age, or leave their parents' houses to try living on their own. Fifty-two is still fraught with romance, adventure, possibilities! At least that's what the media will have us believe, as Japan has entered the era of 人生百年 (jinsei hyakunen, the 100-year life).
A century ago, any Japanese who lived till their 還暦 (kanreki, 60th birthday) was thrown a big party and presented with a 赤いちゃんちゃんこ (akai chanchanko, flaming red vest) as a celebratory token of longevity. And if that person lived another 10 years, they were thrown an ever bigger party for having reached their 古希 (koki, 70th birthday). Koki — short for 古来稀なり (korai marenari, a rare occurrence in ancient times) — is a phrase that's no longer in circulation, though my older relatives like to recall it, laughing to each other that they've become rare specimens.