Original sin is structural Adamic inheritance: humanity is born into the rebellious order Adam established, and each morally responsible person becomes guilty by personally confirming its self-rule.
This is more than a bad example to be imitated, but it is not the mechanical transfer of Adam’s personal guilt as though each descendant had performed his act. Adam’s rebellion established the human order into which we are born: mortal, alienated, without access to the Tree of Life, and structured by autonomy. We inherit that condition; we become personally culpable when we willingly make its rebellion our own.
Infants and others who have not exercised morally responsible agency inherit Adamic mortality, but they have not personally enacted Adam’s rebellion or incurred guilt for his act. Adam’s act constitutes humanity’s inherited condition; our own rebellion establishes our personal culpability.
See in Scripture: Genesis 3:1–24; Romans 5:12–21; Psalm 51:5; Ephesians 2:1–3