  We study and compare equilibrium and aging dynamics on both sides of the
ideal glass transition temperature $T_{MCT}$. In the context of a mean field
model, we observe that all dynamical behaviors are determined by the energy
distance $\epsilon$ to threshold - i.e. marginally stable - states. We
furthermore show the striking result that after eliminating age and temperature
at the benefit of $\epsilon$, the scaling behaviors above and below $T_{MCT}$
are identical, reconciling {\it en passant} the mean field results with
experimental observations. In the vicinity of the transition, we show that
there is an exact mapping between equilibrium dynamics and aging dynamics. This
leads to very natural interpretations and quantitative predictions for several
remarkable features of aging dynamics: waiting time-temperature superposition,
interrupted aging, dynamical heterogeneity.
