Hi Helen, Good question. Now that I think about it, some APPARENTLY old males don't have grey muzzles. But maybe they are just battered and well worn young males??? As for females, I'm not convinced that I could tell an old one by anything other than the grey muzzle.
The definitive thing for ageing is their 'molar progression' but no one has ever done that on live roos. I aged over 1,000 that way but did not record the colour of fur on their muzzle.