That's quite sad isn't it?
I suppose, Frank it's the same reason women's ages are often given, rarely men's: people are described as "pensioners" if over the age of 60 although goodness knows there's a world of difference between the stereotypical little old nana or grandad and most people in that age range 60-90+; hair colour (again for women) "blonde" = ditzy, "flame-haired" = a siren and grey haired - well takes us back to old!
It's trying to establish a persona - however inaccurate or irrelevant.
Nobody describes Helen Mirren as a "grey haired pensioner" even although she is presumably in receipt of her OAP, do they?
But "pensioner- have-a-go- granny" immediately conjures up the image of game old dear bashing yob over the head with her capacious handbag.
It's all a load of nonsense and as usual, the media resort to cliches when they have little of real interest to say.When they have to fill a column they will pad out what litttle they have often with little regard for the truth!