After all, He changed -- He grew from an infant into a Man in His 30's!
Adam was born without sin, but he was capable of corruption, and it was his sin that killed him. If we say that Christ was capable of corruption but not of sin, then He was not just like Adam. Unless He chose not to sin, but could have, just as Adam, then we can's peak of HIm as being "fully man" but a "superman." Just as the Theotokos becomes "superwoman" through Immaculate Conception, and therefore something more than "just human."
Christ, however, became Incarnate using Mary's flesh, which, as we noted, was afflicted by the consequence of the "original sin" (corruption and death), even though she her self did not commit any sin. So, His humanity should have been the post-fall humanity were it not for her to be cleaned of all sin, including the "original sin" — i.e. made "full of Grace" — at one stage in her life, in order to become a fitting vessel. But, if she did not sin since that moment onward, whether it was at her own conception or at the moment of Annunciation, she too would have been immortal! Which may explain why St. John Chrysostomos may have hinted at scriptural evidence that might show that she did sin after being made "full of Grace."
Finally, if God willed to die for His own reasons on the Cross, whereas He would not normally die, then He willfully ended His own life, and this is something we are taught is a sin.
I am sure there are sophisticated writings on this issue somewhere, but the Church hardly ever talks about these somewhat puzzling and open-ended appearances.
"But, if she did not sin since that moment onward, whether it was at her own conception or at the moment of Annunciation, she too would have been immortal!"
This doesn't follow. The Fathers are unanimous in declaring that we must join Christ in death if we are to join Him in the Resurrection.
WHAT!? OK, Forest, deep breaths. :) Do you really think that Jesus was "killed" and did not voluntarily give up His life?
John 10:17-18 : "17 The reason my Father loves me is that I lay down my life—only to take it up again. 18 No one takes it from me, but I lay it down of my own accord. I have authority to lay it down and authority to take it up again. This command I received from my Father."
Do you see what Jesus did as a suicide, or do you see that Jesus was overwhelmed by superior physical forces, taken into custody, wrongly convicted, and executed beyond His ability to stop any of it?