I've just run a test game of 50 years where I haven't done a thing, just kept on checking the age of the oldest chars. There wasn't a single occasion when there was more than 1 person above the age of 80 and there are certainly characters in the books above this age.
In the Feast for Crows Aemon is 102 and Walder Frey is 92. There were other really old characters as well whose exact age I don't know, but to mention some: Jon Arryn, Maester Pycelle or Hoster Tully.
Zirael: I am glad that you have been able to see Aemon reach such a high age, but at my games it has just not happened yet
DavosSeaworth: I have already looked into the game files and I will try to do it, although I haven't made a single mod to the game before