  This paper is part of a series in which we perform a systematic comparison of
the galaxy properties inhabiting compact groups, loose groups and the field. In
this paper we focus our study to the age and the star formation in galaxies.
For galaxies in selected samples of compact groups, loose groups and field, we
compare the distributions of the following parameters: D$_n(4000)$ as an age
indicator, and the specific star formation rate as indicator of ongoing star
formation. We analyse the dependence of these parameters on galaxy type,
stellar mass and, for group galaxies, their dependence on the dynamic state of
the system. We also analyse the fraction of old, and of high star forming
galaxies as a function of galaxy stellar mass in the environments we probe.
Galaxies in compact groups have, on average, older stellar populations than
their loose group or field counterparts. Early-type galaxies in compact groups
formed their stars and depleted their gas content more rapidly than in the
other environments. We have found evidence of two populations of late-type
galaxies in dynamically old compact groups: one with normal specific star
formation rates and another with markedly reduced star formation. Processes
that transform galaxies from star forming to quiescent act upon galaxies faster
and more effectively in compact groups. The unique characteristics of compact
groups make them an extreme environment for galaxies, where the transition to
quiescence occurs rapidly.
