We went last June. Took my Mom 79 yrs young. She has just aslight hip problem and walking on cement all day, would've been agony for her if not for an electric scooter.
We rented an electric scooter from a non Disney company (yes, you can rent them in the parks, but then you have to get to and from your hotel). And at the end of the day it was sooooo nice that Mom didn't have to "walk" back. It was great !
They delivered to our hotel and then picked it up when we left. We didn't have to wait in line each day to rent, so that was a big time saver and it was cheaper.
As to Disney Park rides ....... if there are any that do continual loading, such as HM or Buzz, and you have someone in your group that has a bit of difficulty getting on quickly, just ask the CM to slow it down or stop it and they were very helpful in doing that.
They said just to ask as you were approaching to get on. That it is very common to do so.
We found that out after she fell getting onto a Doom Buggy, while it was in regular loading speed.
Her first ride on that trip was Space Mt.
Loved it !
I guess that all depends on the grandma though. 
you'll have a blast.
