First you need to define Existence.
You could theoretically split existence into two categories. Physical Existence. For example, does my chair exist? certainly it does. Did it always exist as a chair? No, first it started out as a tree. That tree existed though. There are also screwed used to make up the physical existence of my chair. Before they were screws though, they were metal ore found in rocks. Through processing though, when the wood was fashioned into the pieces of the chair, and the metal ore was fashioned into the screws, were combined in a particular way, it gave way to the existence of my chair. It was not a chair prior to the combination of the parts though. The end of physical existence is when your physical being has deteriorated, or has been completely destroyed.
Then you have Conscious Existence. This would be awarded to living entities. Conscious existence would be a little more difficult to determine the moment of existence as you would need to know exactly when in the pregnancy period does the unborn child's brain begin to start making neural connections. This to me would be the beginning of conscious existence. The end of conscious existence would be when brain activity ceases.
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The point I made in my comment above this quote would ask, "But doesn't the vegetable exist as well?"
This would require an additional question. Should we define what the state of existence is at different stages of creation?