In Shakespeare's play, Romeo and Juliet, the reader does know that Juliet is nearly fourteen, so she still has an air of innocence and ingenuousness about her. After all, her father, Lord Capulet tells Paris to wait until she is fourteen to ask for her hand in marriage:
Let two more summers wither in their pride/Ere we may think her ripe to be a bride. (I,ii,10-11)
Being from Verona, Italy, she is probably of Norman descent and, therefore, is fair with light hair (blonde, perhaps) and blue or green eyes. (To this day, many northern Italians are of this coloring.)
As the previous post mentions, the balcony scene contains descriptions of Juliet that enable the reader to envision her. Her eyes must sparkle, for instance, for Romeo says that two stars twinkle in her eyes. Her cheeks are rosy and her skin glowing: "the brightness of her cheek would shame those stars" (II,ii,20). Even in death, Romeo finds Juliet "yet so fair" and "crimson in thy lips and in thy cheeks" (V,iii,95).
Of course, Juliet is reflected in the eyes of the amorous Romeo. Yet, she must be beautiful in a tender way, or else why would Romeo be so enraptured of her so soon after his heartache from Rosalind?