Ferrets don't tolerate heat well at all.  Even temperatures in the 80s
(say, above 27 C or so) can cause problems, and older ferrets can be
even more sensitive.  The first thing to do, of course, is to prevent
heat exposure in the first place, by providing shade and plenty of
cool water.  If you live in a hot climate, you must realize that your
ferret will need special care in mid-summer.  Never leave a ferret or
any pet in a car in hot weather, even with the windows partly open.
It just doesn't do enough good.

There are a couple of ways to keep your ferrets cooler if you don't
have air conditioning.  Fans are an obvious idea, but unless you can
blow in some cooler air, they don't do very much good for ferrets, who
can't sweat.  A plastic bottle of ice wrapped in a towel is helpful.
Finally, you can drape a damp towel over your ferrets' cage, set a
bucket of water on top, and drape another wet rag over the side of the
bucket so one end is at the bottom of the bucket and the other is on
the cage towel.  The rag acts as a wick to keep the towel wet, and the
cage stays cooler from evaporation.

Ferrets in distress from heat will first pant, then go limp, then lose
touch with their surroundings.  The first thing to do is to get the
ferret out of the hot place and start cooling him down slowly.  Cool
water is best, but not too cold, since the ferret's body temperature
will drop way too far, with him unable to stop it.  Anything you can
get him to drink is good, but never force liquids into an unconscious
animal.

After these emergency measures, get your pet to the vet immediately.
Even ferrets that seem to have recovered may die within 48 hours due
to the massive shock they've undergone.  Things to watch for include
tarry stools and vomiting.

On the other hand, ferrets handle cold pretty well.  If they have full
winter coats, they'll be perfectly happy living in a chilly room, say
60 F (15 C).  They can easily handle going outdoors in cold weather,
and many of them love to play in the snow.  Use common sense, though.
Don't take your ferrets out in really frigid (much below freezing) or
wet weather, and bring them inside if they shiver too much, paw at the
door, or try to climb up into your coat.