I believe Im-sama is the person who has eternal life and is the oldest person in One Piece. Here are 4 points to support my theory:
1) Ope Ope no Mi
When I talk about the Ope Ope no Mi, I’m more talking about the Secret Technique to give someone Eternal Life. For the technique to be known, someone in the past must have used it first. But have we seen anyone who possesses eternal life? Im-sama is a possible candidate. Im-sama also has enough power to get someone to sacrifice themselves to give him/her eternal life.
2) Power Vacuum
If Im-sama died, there would be a power vacuum and some confusion over who would control the empty throne. However, if Im-sama has never died, then there has been no power vacuum.
3) Void Century
The Void Century possibly contains information on Im-sama and how the World Government came to be. However I believe that the World Government is trying to cover up the Void Century because of Im-sama as the formation of World Government either by pirates or by evil kingdoms wouldn’t change the current status of the World Government as a global power.
4) The Straw Hat and Joy Boy
This is based off a theory that the Straw Hat at Mary Geoise originally belonged to Joy Boy. And the idea that Im-sama is comparing Luffy to the person who wore the big straw hat in Mary Geoise. If Im-sama didn’t have eternal life, how could he possibly compare Joy Boy and Luffy if he wasn’t around when Joy Boy was alive?
*Theory by AtoZores88