Ferrets should be 3 months or older to get their first rabies shot.  Annual
revaccinations thereafter are recommended for ferrets.  That's fresh from
the Imrab 3 insert.  It probably won't hurt to have it done earlier, but if
you do, have it done *again* at or shortly after the 3 month mark or public
health may decline to recognize the vaccination as valid.
 
I believe the age recommendation is based on their immune system being
mature enough to launch a good immune response at the time of vaccination.
 
-Ilena Ayala
[Posted in FML issue 2759]