We present a model for precipitation kinetics integrating nucleation growth and coarsening. This model allows the description of competitive kinetics of heterogenous and homogeneous precipitation. The precipitation model is coupled to a model for structural hardening predicting the yield stress. This structural hardening model includes the possibility of taking into account the distribution of precipitate size and strength. The model is applied to experimental data on age hardening of Al-Zn-Mg alloys, and allows to describe the effect of a prestraining on the subsequent aging behavior.