I'm currently writing a biography of a fictional character. Should I write "X is a child prodigy" or "X was a child prodigy" if X is currently an adult? My gut tells me to go with a past tense since X is no longer a child, but X is still outstanding in their field, so the prodigy part can still be refered to in present tense.
The answer depends on which tense you use in your narrative. Usually a biography is related from the temporal point of view of the writer's present, so the past events are placed in the past tense:
X was born in 1922 and X began school at the age of 3. By the age of 4, X was posing and answering questions that stumped his teachers. Everyone realized that X was a child prodigy.
But nothing precludes the biographer from adopting the present time of the events:
X is born in 1922 and begins school at the age of 3. By the age of 4, X is posing and answering questions that stump his teachers. Everyone realizes that X is a child prodigy.
Changing the temporal point of view of the narrative changes the tense of the verbs, but of course, it doesn't alter the fact that past events have already happened.
What you can't do is describe the adult X as a child prodigy during the time X is an adult.
If you want to use the present tense, you could say that the person is a former child prodigy.
It sounds clunky, but it's used surprisingly often. There are many "where are they now" articles that use the phrase former child prodigy to refer to an adult who was a prodigy as a child.