I have baseline data for different age groups (6-10, 11-14 and 15-18) for intervention and non-intervention. The project duration is 3 years. I would like to do a difference in difference. However, I can construct only a partial panel as there will be different cases in age groups of 6-10. How do I go about constructing a panel here. Or is it possible to do DID with cross sectional data only?