Interesting discussion.
First, in response to the original question, I believe there is no moral difference between killing the unborn and the born. At any age.
That aside, the lawyer in me wants to nitpick those who say that abortion is murder all of the time. I think someone here already mentioned that “murder is killing an innocent person.” Allow me to expand the definition for the purpose of clarification: “Murder is the killing of a person with with malice aforethought (evil intent).” The person need not even be “innocent” in the way we consider the term.
So, in the case of abortion, yes, it is always killing. And it is always unjustified (regardless of excuse). But my question is this: is there always evil intent involved? Surely, on the part of the doctor performing the abortion this is an interesting question. I would be inclined to argue that most doctors should know, by virtue of their medical training, that the unborn child is a human being (many, if not most medical journals agree upon that point). So, I would want to say that a person who knowingly kills another human being who is a non-aggressor, has at the VERY least, done so unjustly. I would even go so far as to say maliciously. The line between the moral law and the legal one begins to blur together.
But how about the mother? Certainly an abortion clinic is not going to inform a woman about the humanity of the unborn child. Many women, unfortunately, also subscribe the view (sometimes aided by a doctor) that the baby is “not a baby” or “less than human” or simply a “blob.” In these cases, the woman may think that she is not committing an act against human life - she has been duped. Perhaps she should know, but that is a moral question and not a legal one. So is she “guilty” (in the way we would consider a criminal so) of murder? Objectively, she has killed unjustly but cold-blooded murder has a subjective element as well and I’m not always sure that is present in every situation.
But enough of my ramblings - I merely wanted to voice some thoughts.