Influenced by environmental factors, genetics and lifestyle. It can tell you how soon you'll die but not how old you are.
I thought of telomere length too, but given that the person is an outlier in age and they are healthy would it still be as accurate a measure?
I think the biological aspect that is the proxy for age should not affect the well-being of the person.
No it’s not because people are born with different telomere lengths and telomeres do not shorten at the same rate or even at a predictable rate through your lifetime.
Unless that individual's telomeres behave atypically, in direct relation to their longevity?
There is no typical behavior for telomeres. Even in humans they can lengthen under certain circumstances.
They do overall shorten with age but without knowing what was your telomere basepair length at birth for a given tissue (as not only length varies between individuals it varies between different types of cells and between individual chromosomes) and tracking your telomeres throughout your life you can’t get any informative data for a given individual out of it.