Juliet walks a fine line between impetuosity and maturity. Getting married so quickly to an enemy is a rash and ill-considered; remaining loyal to her husband despite adversity might be seen as maturity. Taking the potion is certainly an impetuous act, and though she does seek the advice of adults, she chooses those who give her poor counsel. She says she is not ready to be a wife when her parents want her to look at Paris as a potential suitor, yet she marries Romeo mere hours after saying that. Perhaps it can be said that her emotions are mature though her actions generally are not.
Although I personally think there is more evidence to suggest Juliet's immaturity, if you want to argue that she is actually mature, you need to look at the way that she deals with the chaos that ensues in the play when Romeo slays Tybalt and gets exiled. She is shown to be someone who takes her marriage vows and her loyalty to her new husband seriously, and in spite of her love for Tybalt, she is able to forgive Romeo and love him.
I agree with you that it may be difficult to find quotes to prove Juliet mature. Most readers see her as an impetuous youth driven by lust for Romeo and by defiance for her parents. However, she does handle some rather weighty issues. For instance, on the balcony after her parents' party, she weighs the consequences of Romeo's true identity against her true feelings for him. She says, "What's in a name? A rose by any other name would still smell as sweet." Essentially, she shows maturity here by realizing that Romeo is still sweet to her, regardless of his true name and identity, regardless of her family's hatred for his.
She also forgives Romeo for Tybalt's untimely demise. Her speech before taking Friar Lawrence's potion keeping her safe from Paris so Romeo could come sweep her away from the tomb is also very mature. She weighs all her fears and the motives for why the Friar might really want to kill her or the consequences of the potion working and Romeo not being there to take her away. Her thoughts show both adult insight and childish worries, yet she chooses to trust the Friar and swallows the potion.