Well, based on dialogue with the man, er . . . mer, He has been alive since the height of falmer civilization.
This is well before the Ysgramor and the Atmorans settled in Skyrim. And it was only after the Atmorans were fully settled, did the Dragon Cult rise to power, and in turn, Miraak.\
Now, in ES: Morrowind, we met a Dunmer who was over four-thousand years old, and lived as a Chimer before Azure cursed them into the Dunmer. Up until Skyrim's DLC: "Dragonborn" and "Dawnguard", this Dunmer was the oldest living person in The Elder Scrolls franchise (not including dragons).
Now given that The Dragon Cult fell long before the Chimer were cursed, the evidence so far would suggest that Mirrak is WELL over four-thousand years old. And concidering that the hight of Falmer civilization was well before the Atmorans settled, Gelebor (and his brother) are far older than Miraak, who is far older than the Four-Thousand-Year-Old-Dunmer.
At this point however, his exct age is unknown, and it is unknown if has used magic to prolong his life. All that I do know, is that he is by far the oldest person in the Elder Scrolls.