  White dwarf stars are the most common end point of stellar evolution. Of
special interest are the ultramassive white dwarfs, as they are related to type
Ia Supernovae explosions, merger events, and Fast Radio Bursts. Ultramassive
white dwarfs are expected to harbour oxygen-neon (ONe) cores as a result of
single standard stellar evolution. However, a fraction of them could have
carbon-oxygen (CO) cores. Recent studies, based on the new observations
provided by the {\it Gaia} space mission, indicate that a small fraction of the
ultramassive white dwarfs experience a strong delay in their cooling, which
cannot be attributed only to the occurrence of crystallization, thus requiring
an unknown energy source able to prolong their life for long periods of time.
In this study we find that the energy released by $^{22}$Ne sedimentation in
the deep interior of ultramassive white dwarfs with CO cores and high $^{22}$Ne
content is consistent with the long cooling delay of these stellar remnants. On
the basis of a synthesis study of the white dwarf population, based on Monte
Carlo techniques, we find that the observations revealed by {\it Gaia} can be
explained by the existence of these prolonged youth ultramassive white dwarfs.
Although such a high $^{22}$Ne abundance is not consistent with the standard
evolutionary channels, our results provide sustain to the existence of CO-core
ultramassive white dwarfs and to the occurrence of $^{22}$Ne sedimentation.
