Effect of adopting an age prior (minimum age t > 50 Myr) on the predicted IR luminosity for declining (green, model A) and rising (red, model D) histories. We plot the difference in log (L_IR) between the unconstrained model and the one with age prior as a function of the latter. On average declining SFH show lower L_IRwhereas rising histories predict basically the same, or even higher IR luminosities. Rising SFHs case can thus be distinguished from constant SFR, even if age priors apply.