I would not discriminate someone for her old age. We never know when it is going to be our time to go.. things happen.
I have a clause in my contract mentioning:
The Buyer agrees that if at anytime the dog can no longer reside with her/him, the breeder will be given first option to take back the dog, no monetary compensation is given...
Just in case...
I often forgot, but it is always good to tell the buyers to have a backup plan for the dog in case something happen to them. And to let someone know that the breeder will take the dog back no problems at anytime.
I have told my closest friends and family: When I go.. All my belong will be share between the dogs(I don't have kids)... So you have to adopt one if you want to get anything. They think I am joking... But i am so NOT