Why Is Cherubino Sung by a Female Voice?

Cherubino, the Count's teenage page in The Marriage of Figaro, is played by a female singer. A male part performed by a woman is known as a trouser role. Today Cherubino is often sung by a mezzo-soprano. The important point is not to make the audience mistake the performer's sex, but to build a character through sound.

Start with Voi che sapete in Act Two. Encouraged by Susanna, he sings his own song to the Countess, asking what love is. This is not an adult confidently declaring feelings already understood, but someone pursued by sensations he cannot yet explain.

My listening suggestion is to notice how a singer makes a phrase sound exploratory, rather than simply admiring a beautiful tone. The same flowing melody can sound assured, or hesitant and eager to be understood. Youthfulness does not reside automatically in a voice type: diction, breathing and stage movement have to create it together. An adult singer's technical command can coexist with a character's immaturity.

A trouser role therefore need not be treated as an obstacle to realism. Opera already replaces ordinary speech with singing. Once we accept that convention, the more useful question is what experience this voice makes audible, not whether it resembles a particular teenage boy's everyday voice.

https://www.eno.org/discover-opera/articles/operas-greatest-trouser-roles/
https://wno.org.uk/news/exploring-trouser-roles/