Probably the biggest change after meeting and marrying Romeo would be that her alliance changed from with her family to her husband, which at the time would be a big indication of adulthood.
When Mercutio is slain by Tybalt and Romeo slays Tybalt, Juliet is initially horrified and thinks very ill of Romeo, which the nurse encourages, but when she has a moment to compose herself, she very definitively takes her husband's side over her family's to the dismay and disapproval of her Nurse. In Act 3, Scene 2, Juliet, while conflicted because she loved her cousin makes her loyalty plain:
Will you speak well of him that killed your cousin? NURSE
Are you going to say good things about the man who killed your cousin?
Shall I speak ill of him that is my husband?