[Telomeres may get longer](https://www.ucsf.edu/news/2013/09/108886/lifestyle-changes-may-lengthen-telomeres-measure-cell-aging) if people pick up better eating, execrising, stress management and social support. So it may be hard to tell how long someone has left since they could have a period of time where they slow down, stop or reverse the aging process.
The relation is not completely linear. Some cells might lose all their telomeres at an early age and be replaced with cells that have longer telomeres and some cells might keep their long telomeres throughout the life of the organism. So the reduced telomeres does degrade health as you age but there is no fixed limit for when you die. You might be unlucky and die at 60 years of age from something that you would have survived in your 20s or you could be lucky and live for a hundred years without any major life threatening illnesses. And there is no way to know before hand.
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