  Human languages evolve continuously, and a puzzling problem is how to
reconcile the apparent robustness of most of the deep linguistic structures we
use with the evidence that they undergo possibly slow, yet ceaseless, changes.
Is the state in which we observe languages today closer to what would be a
dynamical attractor with statistically stationary properties or rather closer
to a non-steady state slowly evolving in time? Here we address this question in
the framework of the emergence of shared linguistic categories in a population
of individuals interacting through language games. The observed emerging
asymptotic categorization, which has been previously tested - with success -
against experimental data from human languages, corresponds to a metastable
state where global shifts are always possible but progressively more unlikely
and the response properties depend on the age of the system. This aging
mechanism exhibits striking quantitative analogies to what is observed in the
statistical mechanics of glassy systems. We argue that this can be a general
scenario in language dynamics where shared linguistic conventions would not
emerge as attractors, but rather as metastable states.
