Born a X, reaching adulthood no longer a X, but dead, X.
What is X?
Although alluded to by the use of "adulthood" X was regarded as a living being.
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I think the answer might be
Mute, as in "lacking the power of speech"
So when born, babies lack the power of speech but upon reaching adulthood would no longer be rendered mute. However, dead people lack the power of speech. This word also works well in the sentence structure as it is a noun and an adjective.
Born a mute, reaching adulthood no longer a mute, but dead, mute.