We are using a mixed model to describe trends as subjects age. We have multiple measurements per subject of a continuous outcome. We also adjust for the age at entry into the study. It is of interest if early entry and long follow-up are associated with different outcomes than late entry and short follow-up (i.e. same current age, but entered study at different ages) and so we are including an interaction. So our model is of the form: $y ~ current age + baseline age + current age * baseline age$

Are there any problems with estimation or prediction with a model of this form? I think that this is very similar to the standard age-period-cohort problem, but not sure if that means there are other considerations we need to have when modeling or interpretation.