Did the process of decay and death begin when Adam was created?
To be immortal means to be immune from death. Adam was not immune from death therefore he was not immortal. However, he was not yet mortal at his creation in the sense in which we understand mortality. His condition was "very good" as was the condition of the whole creation.
It wasn't until sentence was passed upon Adam and Eve that the penalty of the law "... in the day that you eat of it [the fruit of the tree of the knowledge of good and evil] you shall surely die" took effect. The Hebrew idiom for "you shall surely die" is "dying you shall die." That is, the process of decay and death began when they chose to disobey God's commandment. Paul tells us in Philippians 2:6 (RSV) that Jesus reversed their sin because he deemed "equality with God a thing not to be grasped." Eve grasped the forbidden fruit and Adam chose to partake of it in order to "be like God". Jesus did not. He humbled himself even to death on the cross. Adam and Eve were punished and humiliated and we suffer the consequence of their error. Jesus was "highly exalted" and we reap the benefits of his humble obedience when we accept his gospel and are baptized to walk with him in newness of life.
Adam was in a unique position at his creation. He was neither mortal nor immortal. His decision would determine his destiny. So will ours.
I hope you have found this helpful.