An Age-Momentum-Correlation experiment, AMOC, was performed with an MeV-positron beam on an amorphous crosslinked poly(methyl methacrylate) (PMMA). The lifetime spectrum can be deconvoluted either into 3, 4 or 5 discrete components. The lifetime subspectra are used to construct the age dependent line shape function S(t), which is compared with the experimental values. Although both 4- and constrained 5-component lifetime analyses have variances X-2(T) approximate to 1.05, the corresponding fittings of the S-curve remain unsatisfactory. Alternatively the lifetime spectrurn was analysed into a set of 3 slow trapping states (ST analysis). In this model p-Ps, o-Ps and e(+) are in the occupied volume at t = 0. During the trapping the S-parameters of the individual states change according to the empirical relation Si = S-0,S-i + S-1,S-i [(1 - exp(- t /tau (S,i))] where tau (S,i) are the trapping times. A perfect fit of the S-curve with X-S(2) = 0.94 was obtained for equal ortho- and para-positronium trapping times tau (S,para) = tau (S,ortho) = (0.54 +/- 0.04) ns.