When we first meet Juliet, she is portrayed as a very young girl. She is in fact so young that she is not yet interested in boys, as we see when her mother asks Juliet what her opinion of marriage is, and she replies, "It is an honour that I dream not of" (I.iii.70). However, when she attends the ball with the purpose of seeing if she can learn to like Paris and meets Romeo, Juliet's perspective of love and marriage very suddenly changes. In fact, she begs Nurse to find out who Romeo is and if he is married, saying, "If he be married, / My grave is likely to be my wedding bed" (I.v.143-44). Her sudden change in perspective of love and marriage shows us that she is maturing from a young girl to a young woman.
Another place in which we see Juliet's maturity is the moment in which Juliet first learns that Romeo has just killed her cousin Tybalt. When Juliet first learns that Romeo has killed Tybalt, her first reaction is to feel utter hatred for Romeo and believe that she had been deceived by him, that he is not what he appeared to be, as we see in the string of oxymora in her lines:
O serpent heart, hid with a flow'ring face!
Did ever dragon keep so fair a cave?
Beautiful tyrant! fiend angelical! (III.ii.76-78)
However, as Juliet regains her faculties and hears Nurse speak ill of Romeo as well, she realizes that Romeo probably killed Tybalt in self-defense and further realizes that, as his wife, she must stand by his side and trust him, even in the darkest moments, as we see in her lines:
Shall I speak ill of him that is my husband?
Ah, poor my lord, what tongue shall smooth thy name
When I, thy three-hours' wife, have mangled it? (102-04)
Hence, we see that this was a maturing moment for Juliet because suddenly she realizes, like a woman rather than a girl, that she can be deceived and that love is not as wonderful as it at first appears to be, but can be rather difficult.