SIn recent years much interest has been attracted by systems which cannot reach thermodynamic equilibrium because of their slow dynamics. Among the most studied there are glassy and disordered systems. Some key features of slow relaxation and aging are also displayed by much "simpler" systems such as pure ferromagnets quenched below or just at their critical point.  In the talk I focus on the field-theoretical approach to determine the relevant quantities (such as the fluctuation-dissipation ratio) associated with the non-equilibrium dynamics of ferromagnetic spin systems quenched at criticality. In particular I compare with recent numerical results and comment on the possibility to define an effective temperature for those systems.