Why would William Shakespeare make Juliet twelve years old? Isn't that a little young to make her fall in love with a fifteen to eighteen year old?

While our society considers this to be inappropriate, it is important to keep in mind that Romeo and Juliet was written for a very different audience. In Elizabethan times, a girl could get married at the age of twelve if her parents gave their consent. While this was not a common practice, it was still considered acceptable. 

In addition, members of noble families married earlier than the lower classes. The Montagues and Capulets would both fit in the noble class. This was to build alliances between powerful families, which allowed the audience to accept the reality of a young girl getting married so early. 

One of the most important factors is that Romeo and Juliet is a Shakespearean tragedy. It is meant to make the audience feel, and hyperbole is one of Shakespeare's greatest tools. Having characters die at such a young age would have a more powerful impact than if they married when they were older. In the end, it is the drama of it all that wins over sense.