In 1996, we took my paternal Grandma to DL, she was 82. She hadn't been since 1969. Back then, the Matterhorn was too much for her, but makes for a funny family story. Alice in Wonderland was her thrill ride on this trip, although I think we might have gotten her on Pirates too.
In 2001, we took my maternal Grandma to WDW, she was 77. Her first ride was Dinosaur. She was a big fan of Fast Track (that would be Test Track for the rest of us, although she also thought Fastpass was a great idea, thus the name confusion). She rode Splash Mountain, and BTMRR. She was wiped out the day we did MGM, so all we did was the GMR and Millionaire, but she would have been happy to go on ToT too. I think RnRC might have been too much though.
It really depends on health, and temperament. My paternal grandma, was not in good health, lots of stiffness, joint pain, heart trouble. But aside from that, she was always worried about her hair (she freaked out on the Jungle Cruise because she thought her hair would get wet), and a bit of a drama queen (I'm so weak and helpless, can you do this for me?). My maternal grandma is a fiesty Italian, raised in Brooklyn, NY during the Depression...no ride was going to best her.
I have to say that I would not trade my Disney visits with my Grandmas for anything. Just like visiting through a child's eyes, nothing can match it.