In Shakespeare's Romeo and Juliet, Juliet is not yet fourteen years old, and she is innocent, having up to this point led a sheltered life in her parents' home. Let's look at some evidence for this claim.
Juliet's parents have determined that Juliet shall marry Paris, who has asked for her hand. Lady Capulet asks her daughter, "How stands your disposition to be married?" Juliet doesn't even know how to answer her. She says she has never dreamed of it. Marriage has, apparently, not been in Juliet's mind. She agrees to her mother's suggestion that she look Paris over at the party that night and see what she thinks of him. At this point, though, Juliet is willing to follow her parents' desires in this matter. She has no real opinion of her own.
When Romeo and Juliet meet at the masquerade, Juliet easily and swiftly falls for Romeo's flattering words and lovely rhetoric. Romeo charms Juliet with his religious metaphor and gets her to kiss him. Perhaps the mystique of hiding behind a mask contributes to this, but Juliet clearly has no real idea how to respond in such a situation. She allows herself to be led and enchanted by Romeo and ends up with stars in her eyes and a kiss on her lips, exclaiming "You kiss by the book." A more experienced young woman (like Rosaline, perhaps) would have put Romeo firmly in his place and not fallen for his smooth lines.
From that point, Juliet believes that she is firmly in love. Yet she really has little idea what this actually entails. Juliet begs the Nurse to "Go ask his name: if he be married." She probably should have found out those details before she kissed him. Then, when she discovers that her slick lover is Romeo Montague, Juliet should be far more in tune with the difficulties of this situation. After all, their families have been in a violent feud for years. Instead, she remarks, "I must love a loathed enemy."
Then Juliet moons over Romeo under the moon, and he woos her in the garden. They agree to marry at once. Remember that Juliet only met Romeo that very evening and that she has already agreed to consider another man. All practicality has left this young lady, and she is much more forward with Romeo than would ever be acceptable by daylight, proclaiming her "true love's passion" for him without once considering the consequences. Juliet is naïve indeed.