In the Thief of Time Wen was trying to find Nanny when she was 'the best' midwife there was for Lady Time. I think one of the times he looked she was 15 and already in full strumpet mode (although the last time I read it there were some undertones that she
may have been molested into that a little to start with...

) - anyway she certainly started collecting notches on her bedpost early so having babies pretty quick too would have been a consequence.
The absence of great-grandchildren, so far as we know, could mean that she's certainly not as old as 80 and if her own children also 'started' early then she might be as young as 40 - don't forget in early industrial eras people aged much quicker, so at 40 by our standards you could look pretty ancient. The 3rd picture
in this post by one of my friends on DreamWorlds - that chap was
42 at the time that photo was taken.
As for Granny - well Terry's sketchy descriptions of her all say she has beautiful, smooth, wart-free skin so given that she's an unmarried witch this doesn't necessarily mean she's that old through magical means. Her skin certainly doesn't match her temperament (and if she was married it would be a whole lot worse I can tell you! :twisted: ). On balance I wouldn't put either of them as too much over 60 years old.