"Genius" and "boy" are both nouns. (Note that the adjective "ingenious" in present English usage is not at all related to the word "genius".) Take some examples:
* Genius scientist
is wrong, instead we prefer
are acceptable, so it seems that "boy" can be used either as a noun modifier (noun-as-adjective) or as a noun.
I don't know if this generalizes to other words in similar categories or not, but I am 100% sure that the accepted and upvoted answer is wrong and thoroughly misguided.