It's very different, so it's hard to give an average age. A few become older than 10 years, the oldest living I know of in Sweden during the beginning of 1997 is 14 years old, but quite a few get ill or are subjected to accidents while young.
Among the youngest accidents seems like the greatest risk, even though some get diseases to, at the ages 2-4 years most remain healthy, but from around the age of 5 some diseases, mainly tumours, start to appear. Many fall victim to them at 5-8 years, which could be said to be a common age for those who survive youth.
The diagram shows the attained ages for 65 ferrets, mostly American, the material gathered by Alan Horne during the beginning of 1997. We can see that 50% die at the ages 5 to 8 years old.