That's so sweet of you~ grandparents are the best, aren't they? I hope you find them a little dog that's just the perfect fit for them!
I had to laugh though...my MOTHER is 76 and if she saw what you wrote about them still being in their own house, still being mobile and doing so well on their own, she'd be AGHAST, ROFL. She's had a few health issues in the past couple of years, but they own 4 houses, she manages 3 as rentals, cleans her own house and yard, drives an hour and a half each way 3 days a week on very heavily congested freeways, goes out to plays and restaurants all the time, drives an hour just to have her hair done, etc.....the only thing she DOESN'T do is get on the computer

. Oh..and she doesn't have a dog b/c she doesn't want to be tied down at all....but she loves her granddogs

and brags about them all the time.
I'm still giggling over it

.
Good luck with your search~ your grandparents are lucky to have you!