[infogulch]

Well he was a child prodigy, but he is no longer a child. A suitable replacement would need to reword the sentence to be about the same length and include that detail without the odd sounding wording.

[chongli]

How about just prodigy?

[irjustin]

For this context, prodigy only applies to children. I'd never call an adult a prodigy except for they were a "former child prodigy".

Somewhere along the line you convert from child prodigy to genius assuming you maintained your ability above the rest of the pack.

[infogulch]

Maybe "was considered a prodigy as a child" has the implications more clearly aligned, though it is a bit longer.