  VAMP (variable-mass particles) scenarios, in which the mass of the cold dark
matter particles is a function of the scalar field responsible for the present
acceleration of the Universe, have been proposed as a solution to the cosmic
coincidence problem, since in the attractor regime both dark energy and dark
matter scale in the same way. We find that only a narrow region in parameter
space leads to models with viable values for the Hubble constant and dark
energy density today. In the allowed region, the dark energy density starts to
dominate around the present epoch and consequently such models cannot solve the
coincidence problem. We show that the age of the Universe in this scenario is
considerably higher than the age for noncoupled dark energy models, and
conclude that more precise independent measurements of the age of the Universe
would be useful in distinguishing between coupled and noncoupled dark energy
models.
