Discuss the ability—the maturity—of Romeo and Juliet to make life-changing decisions.

Juliet, especially, seems to be mature beyond her years.  At only thirteen years old, Juliet is the one who really moves the plot of the drama forward by making some big decisions: it is she, for example, who broaches the subject of marriage with Romeo.  From her balcony, she tells Romeo,

If that thy bent of love be honorable, Thy purpose marriage, send me word tomorrow, By one that I'll procure to come to thee, Where and what time thou wilt perform the rite (2.2.150-153).

Juliet, without prompting from anyone else, plans to send a messenger, the Nurse, to Romeo to see what plans he has made for their wedding ceremony.  At her prompting, Romeo goes to Friar Lawrence to make their plans.  She always strikes me as the planner of the pair.  While Romeo sort of dreamily walks through life in love with her, Juliet is the one making actual strides toward their life together.  Thus, she always seems more mature to me.  
Later, when Romeo kills Tybalt, Juliet finds a way to forgive.  She sends the Nurse, once again, to Romeo to assure him of her continued love.  Further, when her father promises her to Paris, it is Juliet who is willing to undertake Friar Lawrence's frightening plan to drink a potion, appear dead, be interred alive, and finally wake up in her family's tomb.  Romeo tends to follow Juliet's lead.  Even when he takes his life, he does so because he believes her to be already dead.  She is pretty darn mature and seems adept at making big life decisions (though she cannot have made many, if any, in her short life).