As the play progresses, Juliet does develop a more rational mind like an adult's; however, not fully. Juliet lapses back into possessing the irrational, rash, emotionally driven mind of a child when she threatens, and then later commits, suicide.
The best place where we see Juliet develop a more rational adult mind is the scene in which she first learns that Romeo has just killed Tybalt. After Juliet learns of Tybalt's death, she feels that she has been completely deceived by Romeo. She is amazed that a person who can look and seem so wonderful can actually be a devil in disguise, as we see from her string of oxymora found the lines:
O serpent heart, hid with a flow'ring face!
Did ever dragon keep so fair a cave?
Beautiful tyrant! fiend angelical
Just opposite to what thou justly seem'st. (III.ii.76-81)
This reaction of feeling deceived is perfectly natural, but it is also still childlike because it is emotionally driven. It is not until Nurse speaks ill of Romeo saying, "Shame come to Romeo!," that Juliet's mind wakes up and begins rationalizing the situation.
First Juliet rationalizes that she should not speak poorly of a man who is her husband. She then asks herself why Romeo had to kill her beloved cousin and rationalizes that it was because Tybalt was about to kill Romeo. Finally, she rationalizes that she should be rejoicing because Romeo is still alive. We see her rationalizations in her lines:
Shall I speak ill of him that is my husband?
But wherefore, villain, didst thou kill my cousin?
That villain cousin would have kill'd my husband.
Back, foolish tears, back to your native spring!
Your tributary drops belong to woe,
Which you, mistaking, offer up to joy. (102-109)
Since Juliet is rationalizing away her fears and feelings of sorrow in these lines, we see that she is acquiring a more adult mind.
However, Juliet regresses back into the state of an emotionally driven child mind when she decides that the only way she knows how to get out of a marriage with Paris is by committing suicide, an act which strongly contradicts the fervent religious values she demonstrates in the beginning of the play. She regresses again into an emotionally driven child state when she decides that committing suicide is the only way to deal with the misery of losing Romeo to death.