I know that we are all born into original sin, but can an infant commit a personal (actual) sin? I ask this in light of Romans 9:11.
As you acknowledge, King David’s words do apply to all people: “Surely I was sinful at birth, sinful from the time my mother conceived me” (Psalm 51:5). The sinful nature is present when a person’s life begins. As the person’s abilities develop, so do the capabilities of sinning. Some sins might be visible to us; we might not be aware of other sins.
Romans 9:11-13 describes God reversing the birthright blessing, giving it to Jacob instead of Esau. Instead of the older brother receiving that blessing, the younger would receive it. God made that determination before the twins were born and irrespective of any actions on their part: good or bad (Romans 9:11). In other words, the children’s actions did not influence God’s decision.