The following is the author's organization of Kirigakure's original timeline.
There may be some subjective interpretations, so take this as a reference. This story is based on the following inferred timeline.
Terumi Mei was 31 years old during the Fourth Great Ninja War, which took place in Konoha Year 64. Working backward, this means she was born in Konoha Year 33.
Yagura's exact age was never mentioned. Based on Kirigakure's weakened state during the Fourth War, Terumi Mei had likely only been in power for a few years. Considering how devastated Kirigakure was, Obito must have controlled Yagura for at least a decade.
I estimate that, during the events of the original Naruto series (before the timeskip), between Konoha Year 58 and 60, Ao discovered that Yagura was being controlled. Yagura then died, and Terumi Mei took over.
The Three-Tails lost its jinchūriki and later revived in the wild, existing without a host until it was eventually captured by Deidara.
Judging by Yagura's appearance in the Edo Tensei state and his behavior when speaking with Naruto, he was likely no older than 25 when he died.
Taking the highest reasonable estimate, I set his death at Konoha Year 60, meaning he was 25 at the time. This means Yagura was born in Konoha Year 35, making him 7 years old at the time of this story (Konoha Year 42).
He likely became Mizukage after the Third Great Ninja War and was made a jinchūriki between Konoha Year 48 and 50. He assumed the position due to his strength as a perfect jinchūriki.
That would mean he became Mizukage at around 12 years old—similar to Gaara's situation.
(He may have looked like a child, but mentally, he was more like a high school detective... not really.)
If we consider his grandson, Kagura Karatachi, from Boruto, the math looks like this:
25 (age at death in the original series) + 4 (years between original Naruto and Shippuden) + 15 (years between Shippuden and Boruto) = 44 years old.
If Yagura were alive in Boruto, he'd be 44.
Having a 15-year-old grandson at 44 would mean both he and his son had children at 15. Biologically plausible, though not necessarily aligned with social norms or legal restrictions.
Terumi Mei was 27 in Konoha Year 60, making her two years older than Yagura.
Zabuza died in Konoha Year 60 at age 26—one year younger than Terumi Mei, one year older than Yagura.
Kisame Hoshigaki was 29 in Konoha Year 60, born in Konoha Year 31, making him four years older than Zabuza.
Mangetsu Hōzuki, Suigetsu's older brother, has no confirmed birth or death year. I placed him as Yagura's peer.
Ringo Ameyuri's birth and death dates are unknown—she died of illness.
The Third Mizukage, whose age can be inferred based on Ōnoki, was a full generation older than him, placing him in the same age group as the Second Mizukage and Second Tsuchikage. Ōnoki was 79 during the Fourth War, meaning he was 57 at this point in the timeline (79 - 22). The Third Mizukage was at least a full cycle older than Ōnoki, making him about 70 years old at this point.
That covers the major figures of Kirigakure and their timelines.
As a side note, Sasori died in Shippuden at age 35. That means he was born in Konoha Year 29, making him 13 years old in this story.
...
Organizing Kirigakure's Timeline Before and After the Third Great Ninja War
Konoha Year 46–47:
After the Kannabi Bridge incident, Kirigakure sealed the unhosted Three-Tails into Nohara Rin—possibly as part of Madara Uchiha's scheme. Rin later died at Kakashi's hands, leading to Obito's descent into darkness.
The Three-Tails was then sealed inside Yagura.
Yagura became a perfect jinchūriki.
Yagura became the Fourth Mizukage.
Yagura was controlled by Obito.
The exact order of these three major events in Yagura's life is never specified in the original story.
My interpretation is that he first became a perfect jinchūriki, then assumed the position of Mizukage due to his immense power, and was subsequently controlled by Obito.
As for why Obito was able to control a perfect jinchūriki—whether his genjutsu was truly that powerful—I don't really care.
The end result is that he did it.
This marked the beginning of the brutal "Bloody Mist" era.
Zabuza attempted to assassinate Yagura but failed and defected.
Kisame was manipulated into leaving Kirigakure.
The Hōzuki, Kaguya, and many other bloodline and secret technique clans were wiped out.
Kirigakure's high-level combat forces were essentially annihilated, severely weakening the village.
Years later, Ao finally uncovered the truth about Yagura's manipulation. Alongside Terumi Mei and other loyal Mist ninja, he killed Yagura.
At this point, Obito likely considered Kirigakure to be broken enough and abandoned it.
Each Mizukage's Reign and Policies
First Mizukage, Byakuren: From the Warring States era to before the First Great Ninja War. He founded the village on an isolated island in the Land of Water and ruled with an iron fist.
Second Mizukage, Gengetsu Hōzuki: Reigned before and during the First Great Ninja War. He implemented isolationist policies, established the Seven Ninja Swordsmen of the Mist, founded the ninja academy, and created the ANBU and intelligence divisions.
(Whether isolationism started with Byakuren or Gengetsu is unclear.)
Third Mizukage, Jōhei (self-named, original does not specify): The early days of the "Bloody Mist" era.
(It's unknown who exactly started the Bloody Mist policies, but in the anime, Mist ninja referred to themselves as "Blood Mist Shinobi" when they attacked Obito, meaning these practices were in place at least during the Third War, before Yagura officially took power.)
Fourth Mizukage, Yagura: His actions before being controlled remain unknown. After falling under Obito's influence, the brutality of the "Bloody Mist" intensified. Judging by his personality when resurrected via Edo Tensei, Yagura didn't seem like someone who would have enforced the "Blood Mist" policies willingly. If his governance style changed drastically after being manipulated, it would have been fairly obvious to the villagers.
Fifth Mizukage, Terumi Mei & Sixth Mizukage, Chōjūrō: Focused on restoring civilian life. By the Boruto era, Kirigakure had become a thriving tourist island. Chōjūrō may not have been the strongest, but his administrative abilities deserve praise.