Why is dating a solitary red giant using isochrones potentially highly uncertain?

The age result error bar can span a billion years depending on the initial metallicity estimate.

When astronomers attempt to date an isolated, field red giant by matching its color and luminosity to theoretical isochrones on the H-R diagram, the resulting age estimate is often subject to significant uncertainty. This is primarily because the specific evolutionary track a star takes as it leaves the main sequence is highly sensitive to its initial chemical composition, or metallicity. If the initial metallicity of the solitary star is not known with high accuracy—and for isolated stars, this is often difficult to determine precisely—the theoretical isochrone that best fits the observed position can vary considerably. This ambiguity in the initial condition parameter (metallicity) means that the calculated age derived from fitting the post-main-sequence location can easily result in an error margin that encompasses a billion years.