I think the folk definition of aging is actually aligns quite closely with the probability of death definition.

Consider the phrase "He looks like he aged 20 years, but it has only been 5 years since I last saw him." Everybody understands this to mean that he looks like his health has declined in 5 years the way most people's health would decline in 20 years.

If you replace "health has declined" with "probability of death has increased", then it means roughly the same thing. It is true that the folk understanding of "health has declined" could be based on several proxies such as skin texture, baldness, etc. I think what the probability of death definition does is bring some precision to what you mean when you say "health has declined".