I believe it is fairly common for black noses to change to pink with age.
I remember an article on Jeanne Carley's model, Moosephat, who kept his
black nose for a long time and my 4yo Inola has (She's considered a "black"
ferret, anyway)  But the black nose of my 2yo sable is slowly turning pink.
 
Carla, Artie & Emma
[Posted in FML issue 3035]