Expanding my search a bit, I find that in addition to the Guardian and the Elder, there appears to be at least two other Immortals that were/are older than Methos' 5,162 years (dob 3155 B.C.);
Djer an Egyptian Pharoh and Dilijan a Neolithic from the Himalayas. Though I can't find a specific birth date for either, they both seem to be at least 1000 years older than Methos was at that time. The canon is a bit suspect but barring new information, I'll take what I can get at this point.
Reign of Djer - 3100–3055 B.C.E.
At some point in his life, Methos was living with a nomad tribe in Sinai. Distraught at the murder of his wife, he wandered the desert for possibly months until he was caught stealing figs and brought before the Pharaoh Djer. Methos soon realized that Djer was a fellow immortal. Djer, who according to Methos was the oldest immortal (DOB pre 4000 B.C.), planned to offer his head to Methos. Methos decided that he would avenge his wife's death by burying Djer alive. Shooting a poison dart at Djer while he slept, Methos bound him in burial wrappings and placing him in a sarcophagus. After Djer's 'death', Methos became Pharaoh Djet. In roughly 2001, Djer's sarcophagus was recovered and Methos presumably then actually killed him. (DOB pre 4000 B.C. - killed 2001)
At some point during or before the Bronze Age Methos encountered an older immortal named Dilijan. Dilijan lead a band of immortals dedicated to improving the lives of mortals via healing, irrigation, etc. Dilijan's actions sometimes had drastic unintended consequences such as his early separate encounters with Silas and Caspian.
When Kronos turned on Dilijan and slaughtered his followers but Dilijan escaped. Later the Horsemen hunted Dilijan on Kronos' orders. They found Dilijan had created a new peaceful tribe of mortals and slaughtered, raped, and burned the villagers including Dilijan's wife then salted the ground. Again Dilijan escaped. The next time they searched for Dilijan they were unable to find him, eventually they assumed he had been killed. Later Dilijan told Methos that it had been easy to avoid Kronos' clumsy search but if Methos had been interested in finding him he doubted he could have hidden effectively.
Fast forward to 1853
Methos met and fell in love with an opera singer named Violetta under the alias Nathan. Methos confessed a version of his violent past to her. They were together for roughly a year when one of Dilijan's brainwashed followers slipped Violetta a slow acting agonizing poison - Dilijan's long delayed and elaborate vengeance on Methos. After being poisoned Violetta lingered for weeks before finally dying but only after exacting a promise from Methos that he would not kill the man that had poisoned her. She knew that if he went after Dilijan he would revert to the killer he had been before they met.
Fast forward again to early 2010
Dilijan was cornered by Methos who then turned Dilijan's terrible experiments on him as an act of disproportionate vengeance for Dilijan's murder of Violetta. Methos drove so much negative Quickening energy into Dilijan that Dilijan's mind was seared and destroyed, left so full of agony he could not form new memories, a useless quivering hulk of misery Methos condemned him to live. Dilijan is last seen raving and sobbing in a mental institution.
Finally we get to the oldest living 'active' Highlander Immortal:
Dilijan DOB: pre 4000 B.C - Present