Kathleen,
 
We started putting our ferrets on leashes when they were pretty young - 3
1/2 months is probably fine - just don't let them get too cold if you live
in snow country.  I don't know about "training" them to walk on a leash.
Mainly I have them on the leash so that they can explore without
disappearing on me.  Usually I just walk along where they want to go, but
if they head for an area I don't want them in, I simply hold the lead taut,
and after a few seconds they head another direction.  This works pretty
well for progressing up and down the sidewalk too.  I don't know about
anyone else - but I don't think mine will ever "heel" or anything like
that!
 
We also bought a tandem coupler that went at the end of a leash so that we
could walk both ferrets with one lead.  It works OK but having two leads
gives them a little more room to move and explore, and surprisingly my two
don't stick together when outside, they seem to have different interests.
 
Good luck!  Ferrets on leads make great "ambassadors" for the cause and are
a great way to meet new people.
 
Karla
Meep and Daisy
[Posted in FML issue 2552]