“Consider a child below the age of reason. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin. This is indicated by Paul later in the letter to the Romans when he speaks of the time when Jacob and Esau were unborn babies as a time when they “had done nothing either good or bad” (Rom. 9:11).”
A child “below the age of reason” is not the same as Paul writing about Jacob and Esau as “unborn babies” who had “done nothing either good or bad”.
What is your scriptural reference for saying that “sinning requires the ability to reason and the ability to intend to sin”?
“So if Paul’s statement in Romans 3 includes an exception for the Jesus, one may argue that an exception for Mary can also be made.”
If you want to argue this point, you’re arguing with God and His holy word that clearly states all have sinned and fallen short of the glory of God.
Psalms 51
4Against You, You only, I have sinned
And done what is evil in Your sight,
So that You are justified when You speak
And blameless when You judge.
5Behold, I was brought forth in iniquity,
And in sin my mother conceived me.
“David elsewhere speaks of the admirable structure of his body (Ps. cxxxix. 14, 15); it was curiously wrought; and yet here he says it was shapen in iniquity, sin was twisted in with it; not as it came out of God’s hands, but as it comes through our parents’ loins. He elsewhere speaks of the piety of his mother, that she was God’s handmaid, and he pleads his relation to her (Ps. cxvi. 16, lxxxvi. 16), and yet here he says she conceived him in sin; for though she was, by grace, a child of God, she was, by nature, a daughter of Eve, and not excepted from the common character. Note, It is to be sadly lamented by every one of us that we brought into the world with us a corrupt nature, wretchedly degenerated from its primitive purity and rectitude; we have from our birth the snares of sin in our bodies, the seeds of sin in our souls, and a stain of sin upon both. This is what we call original sin, because it is as ancient as our original, and because it is the original of all our actual transgressions. This is that foolishness which is bound in the heart of a child, that proneness of evil and backwardness to good which is the burden of the regenerate and the ruin of the unregenerate; it is a bent to backslide from God.”