AMOC (age-momentum correlation) studies of the slowing-down of positronium in condensed matter have demonstrated that positronium formation does not require special sites (e.g., 'free volumes') and diffusion of thermallized e(+) to these sites. The differences in the energies Eo at which Ps becomes 'permanent' (i.e., does no longer break up into its constituents) between disordered (e.g., organic liquids) and highly ordered (e.g., rare-gas crystals) I's-formers are explained by the different character of the forces acting on the constituents. It is proposed that information on the break-up may be obtained from the relaxation of the positron spin polarization and that very long slowing-down times may be investigated by ACAR-based AMOC measurements.