The shortening of telomeres is a cellular theory explanation of why David is becoming more frail over time.
Telomeres are protective caps at the ends of chromosomes, and they shorten with each cell division. As cells divide throughout a person's lifetime, the telomeres become progressively shorter. This is considered one of the factors contributing to aging and frailty. When telomeres become critically short, cells may become dysfunctional or enter senescence, leading to reduced tissue repair and maintenance, which can result in frailty and age-related decline.