  Energetic particles, such as stellar cosmic rays, produced at a heightened
rate by active stars (like the young Sun) may have been important for the
origin of life on Earth and other exoplanets. Here we compare, as a function of
stellar rotation rate ($\Omega$), contributions from two distinct populations
of energetic particles: stellar cosmic rays accelerated by impulsive flare
events and Galactic cosmic rays. We use a 1.5D stellar wind model combined with
a spatially 1D cosmic ray transport model. We formulate the evolution of the
stellar cosmic ray spectrum as a function of stellar rotation. The maximum
stellar cosmic ray energy increases with increasing rotation i.e., towards more
active/younger stars. We find that stellar cosmic rays dominate over Galactic
cosmic rays in the habitable zone at the pion threshold energy for all stellar
ages considered ($t_*=0.6-2.9\,$Gyr). However, even at the youngest age,
$t_*=0.6\,$Gyr, we estimate that $\gtrsim\,80$MeV stellar cosmic ray fluxes may
still be transient in time. At $\sim1\,$Gyr when life is thought to have
emerged on Earth, we demonstrate that stellar cosmic rays dominate over
Galactic cosmic rays up to $\sim$4$\,$GeV energies during flare events. Our
results for $t_*=0.6\,$Gyr ($\Omega = 4\Omega_\odot$) indicate that
$\lesssim$GeV stellar cosmic rays are advected from the star to 1$\,$au and are
impacted by adiabatic losses in this region. The properties of the inner solar
wind, currently being investigated by the Parker Solar Probe and Solar Orbiter,
are thus important for accurate calculations of stellar cosmic rays around
young Sun-like stars.
