What is the best age to adopt a ferret?

It's not necessarily the best age as much as how the ferret was cared for. Ferret shelters usually have all ages. If you want one that's trained and has calmed down from their baby stage, probably around 1 1/2 - 2 years old, Keep in mind that most ferrets live to about 7 or 8, I had a ferret that lived to 9 years old. Ferrets can be prone to illness and disease from improper care and diets, they also stress very easily which can cause illness. Ferrets in shelters are run by people that are real dedicated and will be straightforward about the ferrets they have and are thoroughly check by a veterinarians