Theatre performances in England's Elizabethan era, when many of Shakespeare's plays were written, were set up differently than in modern times, where men, women, boys and girls of all ages act together on stage. However, in Shakespeare's times, typically only males acted all of the parts in the play.
In Shakespearean times, women were treated differently than men. Things that were socially acceptable for males were not socially acceptable for females. Acting was one of those things. According to the law, women were not allowed to act on stage for any reason. This included acting in plays. Because of this law, someone else needed to play the parts of any women. These roles were often filled by young men for several reasons.
One of the primary reasons that young boys, typically between the ages of 13 and 19, were used to play the roles of women in the Shakespearean era was due to their voices. Boys were cast into these parts when they were at an age where their voices had not yet changed. This allowed the boys to speak in a more normal voice, while still maintaining a higher pitch, thus sounding more like a woman would sound. Once a boy's voice changed, he could no longer play female parts.
The physical characteristics of young boys made them more desirable for female parts within a play versus using an older male to play the part. Young boys lacked the same muscle tone that an adult male would have, thus allowing him to look more like a woman when dressed in the appropriate clothing. With a young boy, facial hair was also not an issue, nor was stature. With a little preparation, the audience would not be able to tell that the young boy was not really a woman from their perspective, though they knew that women were not allowed onstage.
The costumes worn by the young boys were similar in style to what women of the age were wearing. The young boys would be dressed up in fancy dresses and jewellery to set them apart as playing the female roles. In addition, the young boys would also wear wigs and make-up to complete the feminine look. Because wigs were a common staple even among the women, these costumes were not difficult to acquire for the young boys. Once the boys were in full costume, it was often difficult to distinguish that it was in fact a boy onstage.