= Solution
At D, $M_V=20.3-15.09=5.21$. Ignoring differential <Bolometric correction>[bolometric corrections], $L/L_\odot\simeq10^{-0.4(5.21-4.74)}\simeq0.65$, and $L\propto M^4$ gives $\boxed{M_D\simeq0.90M_\odot}$. If accessible fuel fraction is similar, lifetime scales as $M/L\propto M^{-3}$. Normalizing to a solar <main-sequence lifetime> about 10 Gyr gives an age near $\boxed{14\ \mathrm{Gyr}}$.
This is much older than the Sun's current approximately 4.6 Gyr and close to the cosmological age scale. The approximate estimate is not evidence for a cluster older than the Universe: <Bolometric correction>[bolometric corrections], abundance, accessible <hydrogen> fraction and the crude <Mass-luminosity relation>[mass-luminosity] law limit precision. The stated 26.7 MeV conversion and an assumed core fuel fraction can supply the order-of-magnitude solar lifetime; they do not by themselves specify the burned fraction or composition precisely.
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