Chronology of the Hokage Era. (Note: This chapter is a timeline. The first two chapters are a narrative before reincarnation. If you don't like it, you can skip directly to Chapter 2.)
The exact timeline in the original work has never been definitively established, but we can deduce it from the manga world.
(Note: This is calculated based on the original manga, not the anime. Some might ask, 'Isn't the anime valid?' Well, unfortunately, the anime isn't. Because the anime is adapted from the manga and has added many contradictory settings, so the manga takes precedence!)
The following is the timeline divider -----
In Konoha Year 67, the Fourth Great Ninja War broke out and ended in October of the same year (Note: The Fourth Great Ninja War only lasted three days!).
Naruto was 17, Sasuke was 17, Kakashi was 31, Obito was 32 (deceased), and Tsunade was 55.
In Konoha Year 66, Danzo died at the age of 73. In his dying memories, he recalled that 31 years prior, the Second Hokage sacrificed himself to cover their retreat against the Gold and Silver Brothers' unit during the First Great Ninja War.
In Konoha Year 62, Naruto and Sasuke were 12, Kakashi was 26, and Hiruzen Sarutobi died at the age of 69 during the Konoha Crush.
(Hiruzen Sarutobi, the two Advisors, and Danzo were all around the same age. If Hiruzen Sarutobi hadn't died, he would also be 73 in Konoha Year 66.) In the same year, the Three Sannin were 50.
In Konoha Year 57, Naruto and Sasuke were 7. Itachi was 13 when he massacred the Uchiha Clan. Kakashi was 21. Uchiha Fugaku was 40.
In Konoha Year 50, the Nine-Tailed Fox's attack occurred. Obito was 15, Kakashi was 14. Minato and Kushina died at 24! Hiruzen Sarutobi was 57.
Naruto and Sasuke were born in the same year, with Sasuke born a little over a month before Naruto. The Three Sannin were 38. Itachi was 6.
In Konoha Year 48, the Battle of Kannabi Bridge broke out. In the same year, Uchiha Obito 'died' at 13. Kakashi was 12 (Kakashi was promoted to Jonin this year). Rin died at 12. Minato was 22. The Three Sannin were 36.
In Konoha Year 46, Kakashi was 10. The Three Sannin were 34.
In Konoha Year 44, Uchiha Itachi was born. Uchiha Fugaku was 27. Kakashi was 8.
In Konoha Year 36, Kakashi was born.
In Konoha Year 35, the Second Hokage died. Hiruzen Sarutobi, Utatane Koharu, Homura Mitokado, and Danzo were estimated to be at least 42-43 years old. In the same year, Uchiha Kagami died at 25, and Akimichi Torifu at 30. Also in the same year, Uchiha Obito was born.
(Note: Here, the anime created a nonsensical original scene where Uchiha Kagami and the Third Hokage's group were students at the Ninja Academy during their youth, which is very chaotic.)
(When Danzo died, he was 73. He recalled the Second Hokage entrusting his will 31 years prior, which means Danzo was at least 42 at that time. Uchiha Kagami was explicitly stated to be 25 at that time. How could they have been students at the Ninja Academy during the same period?)
Furthermore, the Second Hokage only had three disciples: Hiruzen Sarutobi, Homura Mitokado, and Utatane Koharu.
Uchiha Kagami, Danzo, and Akimichi Torifu were just subordinates of the Second Hokage, not disciples!
Don't be confused by the anime's original nonsense.
In Konoha Year 26, Minato Namikaze and Uzumaki Kushina were born.
In Konoha Year 12, the Three Sannin were born. Hiruzen Sarutobi, Danzo Shimura, Utatane Koharu, and Homura Mitokado were 18.
In Konoha Year 10, Uchiha Kagami was born.
In the early years of Konoha, Uchiha Madara and Senju Hashirama jointly founded the Village.
In Konoha Year -6 (6 years before Konoha's founding), Hiruzen Sarutobi was born.
It's important to note here that Senju Hashirama and Uchiha Madara's ages are not displayed; neither the anime nor the manga specifies them. However, we can make some slight deductions.
Izuna died at 24. Let's assume Tobirama was also 24, the same age as Izuna.
Similarly, let's also assume Madara and Hashirama were the same age.
Hashirama had three younger brothers. The oldest was Tobirama, and the other two younger ones were Itama and Kawama.
Kawama was 7 when he was killed by someone from the Uchiha Clan. Let's assume here that Kawama and Itama were twins, both 7 years old.
Then Tobirama would be around 9 years old, and Hashirama around 15.
Izuna died at 24. In the same year, Uchiha Madara obtained the Eternal Mangekyo at 29. He was defeated in the decisive battle against Hashirama, after which the two clans signed an alliance and established Konoha.
In the early years of Konoha, Tobirama was approximately 25. Madara and Hashirama were approximately 30.
(So, this proves that the Madara Obito saw during the Third Great Ninja War was elderly. Because if calculated this way, in Konoha Year 48, Madara would already be around 78, which perfectly matches a white-haired, aged appearance.)
In Konoha Years 35-36, during the First Great Ninja War, the Gold and Silver Brothers ambushed the Hokage and Raikage. The Second Hokage died covering his subordinates' retreat, at the age of 60-61.
The First Hokage's death was not depicted, but he certainly didn't die shortly after the Valley of the End.
Because both the manga and anime clearly state that the First Hokage met a young Tsunade. At that time, Tsunade was at least 3-5 years old, because the First Hokage recalled young Tsunade learning his gambling habit. (It's impossible for a 1-year-old to gamble, right?)
So, taking the middle value, the First Hokage met Tsunade when she was 4, and then died in the same year.
When Tsunade was 4, it was Konoha Year 16. Assuming the First Hokage founded Konoha at 29-30, then the First Hokage's minimum age at death was in Konoha Year 16, between 45-46.
It's important to note here that Uzumaki Mito might not have been the First Hokage's first wife, because Mito is depicted in the manga as coming to Konoha and becoming the Nine-Tailed Jinchuriki only after the Valley of the End.
Assuming the Valley of the End occurred in Konoha Year 2. Then Hashirama would have been at least 30-31 at that time, which doesn't match the timeline.
So it's possible Hashirama had a previous wife. He married and had children at 20, and then Hashirama's son also married and had children at 20, which would make it possible for Hashirama to see young Tsunade around 44-46 years old.
Tsunade and Nawaki do not have the Senju surname.
Nagato and Karin do not have the Uzumaki surname.
Uchiha Kagami is an ancestor of Uchiha Shisui, not his grandfather.
Indra had the Three-Tomoe Sharingan; the Mangekyo was an anime original. Ashura's Wood Release and Rasengan were also anime originals.
The Six Paths Sage did not have the Sharingan; he was born with the Rinnegan.
As for the ages of some original characters in the anime, I won't deduce them one by one.
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Analyzing the timeline, mainly because some people really don't understand the ages of the Hokage characters, saying things like Hashirama founded Konoha at 50, or Uchiha Shisui was the same age as the Fourth Hokage, or Uchiha Kagami was the same age as the Third Hokage. All sorts of messy stuff.
So here, I'll deduce the age ranges of the main characters based on the manga, going backward, to make it more comfortable for readers. (Note: If you think some parts of Hokage are contradictory, it's definitely because you've watched too many anime original adaptations and read too little manga.)
Without further ado, let the main text begin!