Can someone calculate the age of the KOI-4878 star? The star’s low metallicity and fairly high space velocity suggest that KOI-4878 is older than the Sun.

But I don't know how to calculate an estimation for the age.

KOI-4878 data on Simbad
According to Wikipedia on KOI-4878.01 nobody did yet calculate the age of that object, but says exactly what you quoted:

its low metallicity and fairly high space velocity suggest that KOI-4878 is older than the Sun.

This supposidely relies on the following sources:

https://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=KOI-4878&amp;type=KEPLER_TARGET
https://www.drewexmachina.com/2019/09/05/koi-4878-01-keplers-most-earth-like-planet-candidate/

A search with the keyword KOI-4878.01 on different publication databases like arXiv or scholar.google did not help either.
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