They can live anywhere from 6 to 10 years depending on what resource you read. They are prone to ALOT of health problems. Merlin has insulinoma (pancreas tumors) and an enlarged heart and he's six. Morgan was a test/lab ferret, given the flu to test the flu vaccine when she was less than 2 months old. She now has IBD (like IBS for people) which may or may not be because of the testing...she's 5. The other two are just a year and so far, knock on wood, they are healthy. Most problems show up at 4 years or older.
They are ALOT of fun though and worth it all. Imagine the goofiest pit, shrink it down to 3 pounds and stretch its body out....that's a ferret. THey never grow up, and are balls of energy! They are such a riot and in 4 years, I've not gotten board. They're crazy!!