I usually don't give treats until at a chin is at least 6 months old. Before then, their digestive systems are still developing and just don't handle treats very well. You could end up with a constipated chin, or diahrrea (yuck!)
After 6 months old you can give treats, but only sparingly...NO MORE than one per day, and only give sugary treats, like raisins a few times per week.