Basenjis in the 2004 UK Kennel Club survey had a median lifespan of 13.6 years (sample size of 46 deceased dogs), which is 1–2 years longer than the median lifespan of other breeds of similar size. The oldest dog in the survey was 17.5 years. Most common causes of death were old age (30%), urologic (incontinence, Fanconi syndrome, chronic kidney failure 13%), behavior ("unspecified" and aggression 9%), and cancer. (9%).
Among 78 live dogs in the 2004 UKC survey, the most common health issues noted by owners were dermatologic and urologic (urologic issues in Basenjis can be signs of Fanconi syndrome).<<
That was for a specific study, but if your grandmother has vet records, it would be fantastic for her to send them into her basenji club! I have heard of a few 17 and 18 yr olds, so am sure older are out there like your grandmother's.