Good Question. It prompted me to search and find this interesting read.
Although I enjoy and have studied a wide spectrum of music, my preferred specialty for several years has been Baroque vocal music. Over the last generation or two, singers, directors, and audiences gradually have shown an increased interest in the vocal works written for castrati. Because the era of castrati is long gone, a frequent solution has been to cast such roles with women. These efforts continue to be met with relative success or failure depending upon the quality of the voices and the training of the singers.Not surprisingly, the casual listener conveniently equates high voices with female and low voices with male. If, for example, a male sings with a high tessitura, whether naturally or with falsetto, some listeners fail to analyze what they hear beyond their simple conclusion. I recall during one of my lectures, a music professor and Julliard graduate exclaimed, “Why, he sounds just like a woman!” One irritated response was, “No, he does not!” I suggest that there are two major factors in one’s ability to discern male voices from female voices other than tessitura: musical experience and innate musical sensitivity.
As far as classical theory, there are no specific rules that dictate when and where a male or female should or should not sing a specific part, as long as s/he can "hit the notes." The full read of the article I linked goes more in depth on the differences of timbre. This is the crux of the issue for determining whether to use male or female, child or adult, voices.
What sound are you trying to achieve? What mood are you conveying? Finally, this is art, baby! Some of us males just drop the octave when we're performing a female vocal or singing "Habanera" in the shower...