  A precise determination, and comparison, of the epoch of the onset of cosmic
acceleration, at redshift z_acc, and of dark energy domination, at z_eq,
provides an interesting measure with which to parameterize dark energy models.
By combining several cosmological datasets we place constraints on the redshift
and age of cosmological acceleration. For a Lambda-CDM model, we find the
constraint z_acc=0.76\pm0.10 at 95% c.l., occurring 6.7\pm0.4 Gyrs ago.
Allowing a constant equation of state but different from -1 changes the
constraints to z_acc=0.81\pm0.12 (6.9\pm0.5 Gyrs ago) and
z_eq=0.48\pm0.14(4.9\pm0.9 Gyrs ago), while dynamical models markedly increase
the error on the constraints with z_acc=0.81\pm0.30 (6.8\pm1.4 Gyrs ago) and
z_eq=0.44\pm0.20 (4.5\pm1.0 Gyrs ago). Unified dark energy models as Silent
Quartessence yield: z_acc=0.80\pm0.16 (6.8\pm0.6 Gyrs ago).
