  We have assembled a sample of $\sim$8200 stars with spectral types F5V-M5V,
all having directly measured X-ray luminosities from eROSITA and rotation
periods from TESS, and having empirically estimated ages via their membership
in stellar clusters and groups identified in Gaia astrometry (ages 3-500 Myr).
This is the largest such study sample yet assembled for the purpose of
empirically constraining the evolution of rotationally driven stellar X-ray
activity. We observe rotation-age-activity correlations that are qualitatively
as expected: stars of a given spectral type spin down with age and they become
less X-ray active as they do so. We provide simple functional representations
of these empirical relationships that predict X-ray luminosity from basic
observables to within 0.3 dex. Interestingly, we find that the
rotation-activity relationship is far simpler and more monotonic in form when
expressed in terms of stellar angular momentum instead of rotation period. We
discuss how this finding may relate to the long-established idea that
rotation-activity relationships are mediated by stellar structure (e.g.,
convective turnover time, surface area). Finally, we provide an empirical
relation that predicts stellar angular momentum from basic observables, and
without requiring a direct measurement of stellar rotation, to within 0.5 dex.
