"My child is yet a stranger in the world, she hath not seen the change of fourteen years" What language technique did Shakespeare use here?

In these lines, Capulet tells Paris that as Juliet is only 13 years old ("she has not yet seen the change of 14 years" means 'she is not yet 14 years old'), she is too young and naive to be married. 

The language technique used here is a metaphor, as Capulet calls Juliet "a stranger" to the world. In this way, he compares Juliet's naivety and lack of knowledge about the world to the lack of knowledge we have about a stranger. Over time, a stranger becomes familiar to us, just as over time, one becomes familiar with the world, and naivety is replaced by experience and knowledge. 

Capulet's words are also rather visual and give us a real sense of the passage of time. Juliet has not "seen the change" of many years, and these words give us the sense of passing seasons, or the passage of time. This imagery is reinforced in the following line, where Capulet asks Paris to allow Juliet "two more summers" before she is married.