  We have measured, as a function of the age $t_a$, the aging of the nonlinear
dielectric susceptibility $\chi_3$ of glycerol below the glass transition.
Whereas the linear susceptibility can be accurately accounted for in terms of
an age dependent relaxation time $\tau_{\alpha}(t_a)$, this scaling breaks down
for $\chi_3$, suggesting an increase of the amplitude of $\chi_3$. This is a
strong indication that the number $N_{corr}$ of molecules involved in
relaxation events increases with $t_a$. For $T=0.96 \times T_g$, we find that
$N_{corr}$ increases by $\sim 10%$ when $t_a$ varies from $1\mathrm{ks}$ to
$100\mathrm{ks}$. This sheds new light on the relation between length scales
and time scales in glasses.
