Juliet was very young when she married Romeo. What characteristics of Juliet attracted Romeo enough to want to marry her, besides her youth?
Juliet's beauty is what first attracts Romeo when he sees her at the Capulets' masked ball in Act I, scene V: "Did my heart love till now? forswear it, sight!/For I ne'er saw true beauty till this night." He falls in love at first sight with her, and she with him.
The deeper undertones of their love affair are religious in nature. When they first meet, they speak to each other in a sonnet, calling each other "saint" and "pilgrim." They swear to each other a pure, holy love, far deeper than simple infatuation.