Answering this question depends, in part, on your expectations for a girl who isn't quite fourteen years old. She is in the grip of powerful, new emotions, and the object of her love professes to love her back. How much circumspection and insight would most thirteen-year-olds show in this situation?

She has the self-awareness to consider how her behavior might appear to Romeo, and to speculate that he could regard her own professions of love as evidence that she is easily moved, and therefore changeable:

In truth, fair Montague, I am too fond,
And therefore thou mayst think my 'havior light:
But trust me, gentleman, I'll prove more true
Than those that have more cunning to be strange.

Taking his perspective in this way requires a degree of maturity, though perhaps not a remarkable degree. Then, as now, young girls were warned by their parents and other family members against appearing "easy."

Stronger evidence for "remarkable maturity" comes from her circumspection about the suddenness of their emotions. She warns that their budding romance might be a fleeting, illusory thing. Something that happens so abruptly might end abruptly.

Well, do not swear: although I joy in thee,
I have no joy of this contract to-night:
It is too rash, too unadvised, too sudden;
Too like the lightning, which doth cease to be
Ere one can say 'It lightens.' Sweet, good night!

She hopes this isn't the case, but she shows the insight to recognize the possibility, which requires her to consider her situation from a cooler, more dispassionate perspective. Much older people often fall victim to wishful thinking -- or get so swept up in an intense emotion that they don't think at all. So I think Juliet is showing substantial maturity here.