Now ∼ 15 Gyr is greater than age of the observable universe = 13.797(23) Gyr (Planck 2018) (i.e, the time since Big Bang) according to the Λ-CDM model (which fits almost all observations very well circa 2020).
In fact, all stars ≤ ∼ 0.9 M_☉ probably have main-sequence lifetime longer than ∼ 14 Gyr.???
So such stars have never left the main sequence. All the stars ≤ ∼ 0.9 M_☉ ever formed are still around, relics of earlier generations of star formation. We identify these old stars by their low metalliticity.
The post-main-sequence phase of stars ≤ ∼ 0.9 M_☉ is entirely theoretical since there are none in that phase to observe.
The image shows that stars of 0.1 M_☉ live more than 10**3 Gyr. Those stars will be around a long time.