What did Shakespeare mean by Yea upon thy face Thou wilt fall backward when thou comest to age Will not Julie?

The quote "Yea upon thy face Thou wilt fall backward when thou comest to age Will not Julie?" is from Shakespeare's play Romeo and Juliet. In this scene, Romeo is describing to Juliet how he will fall backward when he comes of age, but he will not fall in love with her. This is because he believes that Juliet is too young and immature for him. He also believes that she is not as beautiful as other women he has seen.