Quote Originally Posted by Monty92 View Post
And why the ruddy hell not? A granny is, by definition, a mother. So why not a f*ckble one?

I don't see any category incongruencies here, unless I'm missing something.
I think that these are very distinct porn categories. In porn terms, a MILF is any woman between 25 and 40 as far as I can make out and thus a reasonably normal object of sexual desire, whereas a 'granny' is an actual old person and thus only of interest to the foulest of dangerous perverts.