How can I describe Romeo and Juliet?

Because Romeo and Juliet is a play rather than a novel,we do not have a narrator giving us an authoritative description of the two characters. Instead, much is left to the creativity of the actors and director; as readers we must use our own imaginations. Because of this, your description should be based on how you imagine these characters as you read through the play.

What we do know is that the play is set in late medieval Verona. We know Juliet to be 13 years old. In the Elizabethan theater the role would have been played by a man, although modern stagings usually cast women in the role. Romeo's age is not specified, though he is described as relatively young. He could be anywhere from his late teens to early twenties. 

Both characters were members of the nobility, from powerful, wealthy families with numerous servants. Neither of them has anything resembling a job. Both seem impetuous, likely to act on impulse without thinking. Both are willing to defy and lie to their families in order to have relationships with people they have barely met, and so we can surmise that they tend to act on emotions rather than thinking and to put their own immediate emotional desires above loyalty to family and friends.