When does estimating age using mass and main sequence position rapidly lose precision?

When a star ages and moves off the main sequence, or when dating very long-lived, low-mass stars.

The method relying on a star's mass and its current position on the main sequence—where it is still actively burning core hydrogen—is most effective for young to middle-aged stars. Its precision rapidly degrades in two distinct scenarios. First, once a star evolves significantly, moving off the main sequence as it becomes a subgiant or red giant, the established mass-luminosity relationships that govern the simpler estimate no longer apply accurately. Second, applying this method to very low-mass stars that reside on the main sequence for exceptionally long periods (many times the current age of the universe) leads to enormous uncertainty. In these cases, even a very small error in measuring the star's initial mass translates into an unacceptably large uncertainty regarding the total age elapsed.