Imu is one of the most important characters in the overarching plot of One Piece. Although Oda introduced Imubquite late into the story, they have gained both importance and popularity quite quickly. In the Reverie arc, Oda dropped a huge reveal for the fans and made it known that at the top of the World Government sits a person named Imu who controls the entire world.
As time passed, more information about Imu became available to the fans. Recently, in the Reverie flashback through Sabo, fans got to see quite a lot about both Imu as well as the Five Elders. Interestingly, an idea was put forward recently that suggested that Imu is actually immortal and has been alive since the time of the Void Century.
The Past Of Imu
Imu is a very intriguing character and it has been essentially confirmed to the fans that they are actually immortal. This was first hinted at during their first introduction when they visited the Underground Chamber in Marijoa and stood in front of a giant Straw Hat that has likely been there for centuries. In the Reverie flashback, through Sabo, it was hinted that Imu was actually the name of one of the rulers of the Founding 20. Furthermore, shortly afterwards, Ivankov confirmed to the fans that Imu was indeed one of the Founding 20 members. A deep dive into Imu's past has not been given just yet, however, fans can piece together bits from the chapters already.
Imu was one of the rulers of the 20 Kingdoms and they hail from the Nerona Family. As expected, the Nerona Family was one of the 20 families that joined forces to destroy the Ancient Kingdom nearly 800 years ago. During this time, the Nerona family was likely not all that special and was just as important as any other family would be among the First 20.
After the destruction of the Ancient Kingdom, Imu and the other 19 would then go on to swear a vow in front of the Empty Throne that they would all be equal. However, Queen Nefertari Lily, who was the ruler of the Kingdom of Arabasta at the time, did not want to become a Celestial Dragon as she wanted to go back to her land and rule her people. For this reason, Lily did not swear her oath, however, Cobra clearly mentioned that there are 19 weapons in front of the Empty Throne, which would mean that Saint Imu actually did swear the vow and put their weapon in front of the throne, meaning they were only as powerful as the other members of the First 20.
For some reason, Imu held a deep grudge against Lily. This good partly be because they suspected her falseness and believed them to be an ally of the people of the D. Their fears were proven right when a "mistake" by Lily led to the Poneglyphs being scattered. Lily also sent a letter to the Kingdom of Arabasta, which has been passed down from generation to generation ever since. According to the letter, Lily asked the people to defend the Poneglyphs and to raise the flag that heralds the Dawn of the world. Lily also proclaimed herself as Nefertari D. Lily, which confirmed the fears of Imu. What kind of relationship they shared in the past is not known to the fans just yet, however, Imu appears to be obsessed with Vivi, one of the descendants of Queen Lily.
How Imu Became Immortal
Although Imu played an important role during the war that was fought during the Void Century, they're still alive right now, which means that they are immortal. In One Piece chapter 1086, this possibility was brought forward by none other than Emporio Ivankov. According to them, Imu of the Nerona Family was one of the members of the Founding 20. However, the fact that they are still alive and sit at the top of the world would mean that they attained the Perennial Youth Surgery of the Ope Ope no Mi. This power was introduced to the fans during the Dressrosa arc, where Doflamingo, also a Celestial Dragon, revealed this to Trafalgar Law and named the Ope Ope no Mi the Ultimate Devil Fruit.
It is through this Devil Fruit that Imu ended up becoming immortal. The fact that this information exists means that someone in the past has done it and is immortal right now. This person is none other than Saint Imu of the Nerona Family. Who exactly was the doctor who performed this surgery on Saint Imu and why isn't known to the fans just yet, however, what the fans do know is that Imu is immortal and likely never has to worry about dying at all. Doflamingo wanted the same thing for himself, however, his plans were ruined by none other than Corazon, his brother.
It can be assumed that one of Imu's closest associates performed this surgery on them in the past. This could be a member of the Nerona Family or another member of the Founding 20 who looked up to them quite a lot. This person would then have died, as the user of the Ope Ope no Mi who performs the Perennial Youth Surgery ends up dying. The Five Elders, who sit at the top of the hierarchy of the descendants of the Founding 20, have, since then, been serving Imu in perpetuity. However, their case is quite curious as well.
The Gorosei's Immortality
Imu might not be the only person in One Piece who is immortal. This immortality could very well extend to the Gorosei as well. Fans have seen them barely age over time and even during their flashbacks that fans got to witness, they look exactly the same. In fact, even 30 years ago, when Vegapunk gained an audience with the Five Elders, they looked exactly the same even though they should be young people at the time and not old figures. This suggests that the Five Elders could also be immortal and this power could very well have been bestowed upon them through either the Ope Ope no Mi, or, there could be another way to bestow immortality to people in the One Piece world that fans do not know of yet.
It is also very important to note that the Gorosei never showed themselves to ordinary people, other than the extremely important ones, such as the Fleet Admiral of the Navy, or characters such as Shanks. They don't leave the Holy Land of Marijoa, so the common world would not know much about them, to begin with. Very few people have ever managed to see them, which means they can very well fool the world into believing that they are just normal humans.
At the same time, their position also indicates the fact that they have been living for quite a while. They are believed to be the Warrior Gods of the world, and, unlike the Ancient Weapons, it is highly unlikely that they are reborn after a generation since they were the victors of the war, to begin with. The Gorosei are surely going to be very important in the Final Saga of One Piece and their immortality could be explained to the fans in the upcoming chapters, when Saturn finally descends on Egghead Island and engages Luffy and Vegapunk in combat.