Well, in this updated film version of the play Romeo and Juliet, Baz Lurhman does what Franco Zeffirelli did before him -- he cast two actors very close in age to the characters of Romeo and Juliet.
The youthful acting work of these teenage actors stands in pretty sharp contrast to the more mature work of most actors cast in theatrical stagings of the play. Both parts are rigorous and demanding for any stage performer, and the actors usually cast to play them onstage are in their early to mid twenties. Sometimes, actors in their thirties have played these roles!
In Shakespeare's time, the roles would, most probably, also have been played by teenagers, but both parts would have been played by teenage boys. So, both Zeffirelli and Lurhman are closer to the original staging with their casting choices.
Shakespeare created two very unique characters in Romeo and Juliet, in that it is Juliet who speaks and behaves as a young male lead character should (speaking directly and taking courageous action) and it is Romeo who speaks in flowery, romantic verse and is much more reluctant to initiate action.
So, the main choice Lurhman makes in his film Romeo + Juliet is to cast young actors in the parts. In this way, the youthful impulses of Leonardo DiCaprio and Claire Danes can shine through these teenage star-crossed lovers.