All parts in Elizabethan theatre were played by men. Female parts were taken by young boys, preferrably by boys whose voices hadn't broken yet. The roles of older women, particularily comic ones, may have been played by men. (Brockett 1991: 165) Women didn't act in English theatre during the entire golden age of Elizabethan theatre and up until the civil war in 1642.
Live Hov speculates that Shakespeare's female roles might have been richer if he'd had experience with female actors. The boys who usually acted the womens' parts grew up quickly and would have had little time for artistic development. The age of the boys who played the female parts may also have influenced Shakespeare to portray mainly young women. Perhaps good actresses would have encouraged him to create more mature women, like Lady Macbeth and Cleopatra. On the other hand, Shakespeare was influenced by Italian drama and theatre, and despite the actresses in Italy at the time, most of the female parts were young women.
Another possibility Hov suggests is that putting young boys in womens' roles may have been an erotic refinement quite enjoyed by the audience. (Hov 1990: 118-120)