We sometimes confuse two related but separate concepts. We need to make a distinction between
- the commission of a sin, and
- the possession of sinful flesh.
The Bible is very plain from beginning to end that sin is an act committed by the individual sinner. It states explicitly, for example, that a person is not counted guilty for the sins of his ancestors. That means, among other things, that we are not guilty of Adam's sin (contra Augustine) even though we inherit the results of his transgression. If we say that Jesus was born as we are (which I believe) then sinful flesh cannot be sin per se. If we hold that we are guilty of Adam's transgression (the doctrine of original sin), then the only logical solution is some variation of immaculate conception which makes Jesus fundamentally different from us.
So when does a baby become a sinner? It is born with sinful flesh, as Jesus was, but becomes a sinner when it sins--as all babies (except Jesus) in the history of the human race did.
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