In Doctor Who, before the Time War, who was the oldest living Time Lord in existence?
I nominate Rassilon, who according to the TARDIS Wikia, is one of the founders of the Timelord society:
Rassilon was, alongside Omega and the Other, one of the founders of Time Lord civilisation and widely regarded as the single greatest figure of Gallifreyan history. He was generally considered the first Time Lord, though some believed that distinction belonged to his compatriot Omega.
Considering he was still around during the events of Hell Bent (Series 9 finale), and Omega hasn't been seen in New Who (yet), he must be the oldest living Timelord.
The three oldest Time Lords were Rassilon, Omega, and the other the oldest of them presumably being Rassilon and the other (childhood friends) then Omega their age is unknown but it has subtle hint they say the doctor could be the other reason being in the episode Terror of the Autons of the classic Doctor who you get to see all of the Doctors previous forms it goes from three to two to one but it keeps going none look familiar though so these must be regenerations from before he left his home but as it goes back you see face after face until you see a shadow figure with no visible features but this is still a form of the Doctors but it is said the Doctor came in shortly after the other disappeared and no one ever knew what the other looked like or how old he was they just knew he was there from the beginning and was an advisor of Rassilon so there you have it I believe the Doctor and the other are the same person making the Doctor the oldest Time Lord.