  It has generally been assumed in the literature that while young supernova
remnants (SNRs) accelerate particles even in the early stages, the particles do
not escape until the start of the Sedov-Taylor or adiabatic stage, when the
maximum energy of accelerated particles is reached. These calculations however
do not take into account the detailed hydrodynamical expansion in the
ejecta-dominated stage, and the approach to the Sedov stage. Using analytic
approximations, we explore different environments in which the SNR may evolve,
and investigate how the maximum energy to which particles are accelerated, and
its time evolution, depends on various parameters. We take into account the
ambient magnetic field and its amplification by resonant or non-resonant modes.
Our studies reveal that the maximum energy to which particles are accelerated
is generally reached in the ejecta-dominated stage, much before the start of
Sedov stage. For SNe evolving within the winds of their massive stars, the
maximum energy is reached very early in the evolution. We briefly explore the
consequences for supernova remnants expanding in surroundings such as
wind-bubbles or superbubbles.
