We have found that pre-kindergarten children (age 4) and kindergarten children (age 5) are able to follow picture-book stories centered on family love. This is an important foundation for learning how to forgive. We have found through our scientific studies that children as young as 6-years-old can understand the causes and consequences to behavior. They, therefore, can understand unjust actions by others (a cause) and the development of resentment in the offended person (a consequence). Further, these 6-year-old children then can understand that the resentment can be overcome by forgiving, which in some cases can restore relationships (if the other is willing to cooperate).
For additional information, see Your Kids Are Smarter Than You Think.