I would sell to someone that age if they seemed fit and healthy. My dad is 94 and could easily take care of a dog. If he had gotten a dog at 80, it would be 14 y/o now!!
He works every day (volunteer) preparing meals at the food kitchen. There are vast differences between people. I have met some 40 year olds that seem very old for their age and I have seem people like my father, who are full of energy and love life.
But, life is fragile at that age, so I do understand your concerns and think that a clause in the contract stating that the dog is returned to you (or to a family member that you like and approve of) should something happen is a good idea.