How old is Deckard Cain?
It is unknown, along with the ages of most Diablo-series characters.
Deckard Cain's age is a popular question due to his renown in the series, but remember that the three-game timespan is only 27 years, a reasonable mortal timeframe. He notes that he is near the end of his life1 in the foreword and afterword to his Book of Cain, which prefaces Diablo III. If Cain is mortal and about to die, consider that a "Middle Ages" setting's life expectancy is ~70 years max,2 which puts him near ~40 at the beginning of Diablo I (1258).3
Cain and the Horadrim do not live extended lifespans. For the record, Jered is Deckard's ancestor, and not his father. As per the Book of Cain, there are many generations in-between Jered Cain of 1019 and Deckard Cain of 1285:
...from writings passed down through my family from my ancestor Jered Cain... (p. 88)
So chalk up his Sean Connery doddering voice to the "wise old man" trope sounding older than he looks. Blizzard consciously withholds Cain lore for the good of the series. If you want the truth, though, the real Deckard Cain Elder is 16.4
The real age of Cain in unknown,there are only estimated ages depending on the age of known people and major events.
Cain is the descendant of Jared; most people think he is his son and they make Cain ~300 years old.
Here is the estimated longer "family tree" (source),40 years is used as a step which is probably even less:
- 1020 Deckard's great great grandfather is born (son of Jered)
- 1060 Deckard's great grandfather
- 1100 Deckard's grandfather(Jared Cain’s Horadrim troupe captures Diablo)
- 1140 Deckard's father
- 1180 Deckard Cain is born
- 1285 Diablo 3
This puts him on 105 which is possible even in real aging,other way Jared was the leader of the Horadrim and to become that he could have been even older than 140 years, which would say that all Horadrim tend to live more.