  Observational constraints to a large class of decaying vacuum cosmologies are
derived using the angular size data of compact radio sources and the latest age
estimates of globular clusters. For this class of deflationary $\Lambda(t)$
models, the present value of the vacuum energy density is quantified by a
positive $\beta$ parameter smaller than unity. In the case of milliarcsecond
compact radio-sources, we find that the allowed intervals for $\beta$ and the
matter density parameter $\Omega_m$ are heavily dependent on the value of the
mean projected linear size $l$. For $l \simeq 20h^{-1} - 30h^{-1}$ pc, the best
fit occurs for $\beta \sim 0.58$, $\Omega_{\rm{m}} \sim 0.58$, and $\beta \sim
0.76$, $\Omega_{\rm{m}} \sim 0.28$, respectively. This analysis shows that if
one minimizes $\chi^{2}$ for the free parameters $l$, $\Omega_{\rm{m}}$ and
$\beta$, the best fit for these angular size data corresponds to a decaying
$\Lambda(t)$ with $\Omega_{\rm{m}} = 0.54$ $\beta=0.6$ and $l = 22.64h^{-1}$
pc. Constraints from age estimates of globular clusters and old high redshift
galaxies are not so restrictive, thereby suggesting that there is no age crisis
for this kind of $\Lambda(t)$ cosmologies.
