Although I don't know just how old mine are, I know they are at least
a couple of years old, and I was able to litter "train" them.  I
incorporated used litter from their missing litter trained brother in
their litter pan, and the scent helped them catch on.  Scent plays an
important part in where they poop, and once a scent is established,
it's important to not strip it completely away by washing the litter
box too much (or at least save some used litter and put it in the pan).
 
HTH,
Gary
[Posted in FML issue 4313]