I have a hobby of tracing family trees, and one day I decided to trace royal family trees. I started with the British, then the French, then the Spanish. Now I am tracing the Japanese Emperors. I use Wikipedia and Google for the family trees, and I saw that the earliest Japanese emperors had lifespans going beyond Jeanne Calment (the oldest verified person at 122 years old). For example, the first Japanese Emperor, Jimmu, died at the age of 126, and Emperor Suinin was reportedly 138. Is this true?
In short, no. Emperor Jimmu and many of the early emperors are certainly mythological.
The records of what we have for Japan's earliest emperors come from two sources, the Kojiki and Nihon Shoki, which were both commissioned by the Imperial Family in 711-12 CE and and 720 CE, respectively. These documents were propaganda pieces meant to legitimize the Imperial Family by drawing direct descent from the gods through Emperor Jimmu. There are purported earlier documents, namely the Tennōki written in 620 CE, but there are no extant copies and there's no indication it would have had any better records of the earliest emperors. Given Emperor Jimmu's supposed birth date of 711 BC, that puts 1300 years between the supposed events of Jimmu's life and when the records were first written down.
The records of Emperor Jimmu are full of myths and implausible details, such as the descent from the Sun Goddess Amaterasu and the presence of a three-legged crow, alongside the long lifespan you mention. Archaeologically as well, we have no evidence of a centralized Japanese state until into the Kofun era (3rd century to 6th century CE), hundreds of years after Jimmu. In fact, the Chinese record Han Shu compiled in the 1st century CE briefly mention Japan as having a variety of disparate, small polities.
The earliest historically verifiable Emperor is Emperor Kinmei, who lived from 509 CE to 571 CE. There are inscribed swords, the Inariyama Sword and Eta Funayama Sword, which might mention Emperor Yūryaku (456-479 CE), but that also doesn't get us much further back. An early Chinese record of Japan, the Wei Zhi from 297 CE, famously makes a mention of the shaman queen Himiko of Yamatai (189 CE-248 CE). Whether Himiko corresponds to any figure in the Kojiki or Nihon Shoki, or whether Yamatai corresponds with Yamato (a region of Japan which became the political center and was later used to describe the entire country) are still debated and unlikely to yield any definitive answer.
So, the earliest verifiable Emperor is Kinmei in the 6th Century CE, possibly Emperor Yūryaku in the 5th Century CE, and a very tenuous connection to Queen Himiko in the 3rd Century CE. Beyond that, there's really only myths and speculation.
Sources:
Nihon Shoki, Kojiki, Wei Zhi
Brownlee, John S. Political Thought in Japanese Historical Writing : From Kojiki (712) to Tokushi Yoron (1712) (2006)
Imamura Keiji, Prehistoric Japan : new perspectives on insular East Asia (2003)
