Dr. Bruce Williams, DVM, says:

    I know that some practicing vets consider a 3-year animal to be
    "geriatric" and to require a CBC [complete blood cell count] and a
    fasting blood glucose yearly, but as one who stands to make no
    money on this deal anyway, I don't recommend it until age 5.
    Three years is just too young to consider a ferret geriatric.

    Now, remember, all ferrets are different.  If you have one that is
    sort of "puny", never eats well, sluggish, etc.  a yearly CBC and
    glucose is a good idea every year.  But if your three or 4 year
    olds are healthy, well, then it's just not required.  I start mine
    at 5 years.

    Considering dental work - have your vet check the teeth and then
    recommend who needs it.  Not every ferret will need to have it
    done, and if your 4 year olds have been on hard food all of their
    lives, chances are good that they may not need any work yet.

    Remember - a healthy 3- or 4-year old doesn't necessarily require
    any annual bloodwork, but a sickly 2 year old should get it on at
    least an annual basis.