I don't think there is lore for any Worgen dying from old age. All the Gilneans who have been turned to Worgen but not actually "cured" are still running around Silverpine. The original druids who were lost to Goldrinns curse are still alive, which is saying that the curse does nothing to shorten an elves life.
Chances are Genn is cursed to be 70-80 years old until he is actually killed.