If we are looking at just the themes surrounding the two star-crossed lovers, Romeo and Juliet, rather than looking at the play's themes as a whole, there are definitely a few different themes.
One theme surrounding Juliet is coming of age. Juliet is a budding young woman. When we first meet her, she appears to be far more young and girlish than in later scenes. One evidence of her youth is the fact that when her mother asks Juliet what her opinion of marriage is, she says, "It is an honour I dream not of," showing us that, like a young girl, Juliet is not yet really thinking of boys (I.iii.70). More evidence that Juliet is mentally much younger in this scene is that she is still very eager to please her mother and play the role of the obedient daughter. We see her playing the role of the obedient daughter when she consents to take a look at Paris at the ball and see if she can like him. However, as the play progresses, Juliet experiences tribulations that help her develop into more of a young woman. First, she falls in love for the first time; then, Romeo kills her cousin Tybalt. The emotional turmoil she experiences after Tybalt's death, her feelings of both love and hatred for her husband, plus her choice to continue loving her husband show us just how much she has developed into a young woman.
Another theme is rash, irrational youth, which is especially portrayed through Romeo, though both Romeo and Juliet have their rash, irrational moments. We particularly see Romeo acting rashly and irrationally when it comes to love. For one thing, he allows himself to get caught up in his emotions rather than using sensible judgement. He gets caught up in his emotions both when he allows himself to suffer over Rosaline's rejection and when he falls in love with Juliet. When he gets caught up in his emotions, he acts irrationally. He also makes the hasty decision to marry so suddenly, even though both he and Juliet know they have a lot of obstacles, such as their warring families. Making such a sudden decision is acting rashly. Finally, Romeo shows just how young he truly is when he confuses real love for lust. Friar Laurence expresses this truth when he observes, "Young men's love then lies / Not truly in their hearts, but in their eyes," in other words, all Romeo is really feeling for both Rosaline and Juliet is physical attraction and nothing deeper (II.iii.68-69).