A person who is addicted to mature/vintage/oldschool porn. That is porn of the saggy and grey pubed variety. Each man to his own, but when found on his/her computer they do nothing to cover it up, in fact telling the audience what picture is coming up next on the slideshow, he/she admits that is wrong and tries to blame the internet, but still knows exactly whats on there and still does not delete it.
The grannyrider term originates after Top Gun callsigns, when choosing who we each were in the movie, said granny porn lover, wanted to be Iceman, to which the reply was "Negative Grannyrider, the pattern is full", sypmtoms of being a grannyrider include smackeye, lack of friends and general abuse from the public.
There is no cure of Grannyrider, once dubbed a Grannyrider, that name will stick with them forever, the Grannyrider will lose all other nicknames as this one takes priority.
Ross:"I am Iceman"
Lou:"Negative Grannyrider, the pattern is full"
"Grannyrider are you looking at your contraversial porn again?"
That kid looks like he's got a touch of smackeye, bet me he is a Grannyrider and buzzes off old school porn.