The relaxation of the low field thermoremanent magnetization of an Fe 0 . 6 5 Ni 0 . 2 4 Cr 0 . 1 1 ferromagnet has been measured, as a function of temperature and system age, over four decades of observation time 1 s < t < 10 4 s. Above T g 25 K, the relaxation dynamics are stationary, and below T g , they are glassy, and the response curves have age-dependent inflection points. Fits to the predictions of a model of diffusion among random traps indicate that the dynamic crossover is correlated with a loss of ergodicity below T g due to a change in the structure of phase space.