I don't know enough to argue with the medical theory behind early
spay/neuters being a bad thing.  All I can relate is my own experience,
which is admittedly limited.
 
I didn't neuter Sammy till he was a year old (I even brought him to a
breeder where he fathered 3 litters).  Sammy was a vigorous, healthy
young boy, and big at 4.25 pounds.  At that point my only other male,
Buddy, was 3 pounds (don't know if he was an early neuter; got him as
an adult).
 
Being neutered later didn't spare Sammy any of the usual health problems.
He had adrenal disease and insulinoma.  Plus he had eye problems early,
with glaucoma later, and liver failure late in life.  He lived to 7 years
and 4 months age.
 
I've since had two other males.  Both were early neuters, but it didn't
stunt their growth at all.  They were as big as Sammy, and I'd never met
anyone else with as big a ferret as Sammy (I've heard people on the list
with 6 pound ferrets.  I just know everyone who met my guys thought they
were *big*).  Buster sadly died at 9 months of lymphoma, but Tommy is
still with me at 15 months age (so is Buster's sister Pandora, who will
be 2 next month).
 
By the way, in case anyone's wondering, a whole male ferret is *not*
fun to have around.  When Sammy was in season he smelled about 10 times
stronger, left stinky slime trails everywhere to mark his territiory,
and could not be kept with the other ferrets since he would attack them
(though he was still sweet as ever with me).
 
--
John Rosloot, Caregiver to Pandora and Tommy
With loving memories of my Big Buster Boy
my dear departed Buddy,
my precious little Cassie-angel,
and my Silly Snuggly Sammy
http://www.cs.uregina.ca/~rosloot/ferrets
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[Posted in FML issue 4133]