  A dynamical model of the decaying vacuum energy is presented, which is based
on Jordan-Brans-Dicke theory with a scalar field $\phi$. The solution of an
evolutionary differential equation for the scalar field $\phi$ drives the
vacuum energy towards a cosmological constant at the present epoch that can
give for the age of the universe, $t_0\sim 13.5$ Gyr for $\Omega_0=1$, which is
consistent with the age of globular clusters.
