Well, considering the dangers of the world in general and the general lack of health care in the world, I would say that the average life expectancy of  is around 32 years old. Which, is the same life expectancy of medieval china. So, any older than forty would not be hard to do but the characters are more than likely not going to have the safest or healthiest lives. Also considering that bandits are a common sight, characters aren't likely to make it much further in life than fifty years. Considering that no character is perfect in a fight, getting injured is nearly a sure fact. So, I would consider it pretty lucky to live past the age of forty to forty-five. Heck, even Shogun Izami, with the strongest Shogunate overall, is only fifty-three years old and his father died at the age of thirty-eight.