  Nearby associations of stars which are coeval are important benchmark
laboratories because they provide robust measurements of stellar ages. The
study of such coeval groups makes it possible to better understand star
formation by studying the initial mass function, the binary fraction or the
circumstellar disks of stars, to determine how the initially dense populations
of young stars gradually disperse to form the field population, and to shed
light on how the properties of stars, exoplanets and substellar objects evolve
with distinct snapshots along their lifetime. The advent of large-scale
missions such as Gaia is reshaping our understanding or stellar kinematics in
the Solar neighborhood and beyond, and offers the opportunity to detect a large
number of loose, coeval stellar associations for the first time, which evaded
prior detection because of their low density or the faintness of their members.
In parallel, advances in detection and characterization of exoplanets and
substellar objects are starting to unveil the detailed properties of extrasolar
atmospheres, as well as population-level distributions in fundamental exoplanet
properties such as radii, masses, and orbital parameters. Accurate ages are
still sparsely available to interpret the evolution of both exoplanets and
substellar objects, and both fields are now ripe for detailed age
investigations because we are starting to uncover ever-closer low-density
associations that previously escaped detection, as well as exoplanets and ever
lower-mass members of more distant open clusters and star-forming regions. In
this paper, we review some recent advances in the identification and
characterization of nearby associations, the methods by which stellar ages are
measured, and some of the direct applications of the study of young
associations such as the emergent field of isolated planetary-mass objects.
