We examine the isotope production in star forming regions through a model of stellar-group population synthesis evolution. From this we obtain the light-curves of γ-ray line emission due to radioactive decay of 26Al, 60Fe and the e -e + annihilation line. We discuss in particular, the effects of the dispersion due to the discreteness of the stellar populations. We conclude that when predicted γ-ray line observations are combined with other multi-wavelength measurements, one can efficiently constrain the age of a stellar population, and help to identify the primary nucleosynthesis sources of the radio-isotopes.