[timgilbert]

I'm not sure that Shakespeare really had a concept of beyond young infatuation like we have today

I'm not sure what you mean, but if you mean that he mostly thought of love as childish infatuation, you can see him exploring quite a variety of other types of love in the sonnets.

[bazoom42]

The love of Romeo and Juliet is certainly more mature than say Othello and Desdemona, or Hamlet and Ophelia, or Antony and Cleopatra. So I think “childish” is coming from prejudice, not from the play itself.

Btw the age of Romeo is never stated. Juliet is very young, but her parents are consistently depicted as much more childish and immature.