  The observational signature of core crystallization of white dwarfs has
recently been discovered. However, the magnitude of the crystallization-powered
cooling delay required to match observed white dwarfs is larger than predicted
by conventional models, requiring additional mechanisms of energy release in
white dwarf interiors. The most ideal benchmarks for understanding this
discrepancy would be bright and nearby crystallizing white dwarfs with total
ages that can be externally constrained. In this work we report that a recently
discovered white dwarf is a bound companion to the triple star HD 190412,
forming a new Sirius-like system in the solar neighbourhood. The location of HD
190412 C on the $T_{\text{eff}}-\text{mass}$ diagram implies it is undergoing
crystallization, making this the first confirmed crystallizing white dwarf
whose total age can be externally constrained. Motivated by the possibility
that a cooling delay caused by crystallization can be directly detected for
this white dwarf we employ a variety of methods to constrain the age of the
system; however, our empirical age anomaly of $+3.1\pm1.9$ Gyr is ultimately
too imprecise to reach statistical significance, preventing us from making
strong constraints to models of white dwarf crystallization. Our results are
nonetheless compatible with the recent hypothesis that $^{22}$Ne phase
separation is responsible for the excess cooling delay of crystallizing white
dwarfs. The discovery of this system at only 32 parsecs suggests that similar
benchmark systems are likely to be common; future discoveries may therefore
provide powerful tests for models of white dwarf crystallization.
