The Five Elders are a group of the highest-ranking Celestial Dragons that make up the head of state for the World Government. They essentially hold the greatest authority over the entirety of the world; the only exception being the mysterious sovereign Im whom they answer to.
Their influence over the World Government and its military is paramount and limitless, including command over the Marines, Cipher Pol, and even the Seven Warlords of the Sea prior to their abolishment, making them one of the main antagonistic groups of the series.
So far, it is unknown how they became the leaders of the World Government or how long they have been ruling it. They are almost always found in the Room of Authority in Pangaea Castle.
There is a certain theory that has been around for a while now and has also known many different ways of interpretation and explanation. The basic concept of the theory revolving around the Five Elders, is that there is a possibility that they are immortal or at least have an incredibly long lifespan that could potentially date back to even the Void Century.
The believers of this theory usually see the Gorosei as the people who were there when the World Government formed and took control of the world, and they have likely been in command of the organisation for all these centuries. Because of this, it is likely that they might serve as the final antagonist of the series, due to their mysterious and likely dark past during the events of the Void Century which has ties to many aspects of the series.
While there is hardly any evidence for this specific theory, there is one thing that many fans take as the prime piece of evidence. During Robin’s flashback of the destruction of Ohara, the Five Elders are briefly shown during the whole ordeal. During their appearance, they seem to have the exact same appearance that they have in the current storyline. Although their appearance is the same and they don’t seem to have aged at all during this period of time, the events of the Ohara flashback and the current storyline are 20 years apart. Because of this, there is a lot of confusion and discussion about the aging process of the Five Elders and the possibility that they could be immortal.