What if a more useful definition of age isn't your solar revolution count, but (mitochondrial function, concentration of tau in your prefrontal cortex, degree o

What if a more useful definition of age isn't your solar revolution count, but (mitochondrial function, concentration of tau in your prefrontal cortex, degree of autoimmune attack on your joints, reaction time, etc.)? Then age starts to look as definitional and mutable as names or gender. Why not petition to change it? Anecdotes: I know someone age 53 whom I would not trust to add two numbers and get the right answer. OTOH, my wife's uncle, who died two years ago at 91, was somebody I consulted regularly until his death about MacOS issues and what DSLR I should use to photograph microwave plasmas. He was an MD. Go figure. I think definition of age deserves more discussion.