In the first scene of the play, Romeo is described by both Benvolio and his father as behaving lately in a worrisome manner. Both Benvolio and his father are concerned because they have repeatedly seen Romeo up at dawn and with tears in his eyes and neither of them know what Romeo is feeling distraught about. Therefore, the best lines early in the play that describe Romeo, are the ones that describe his worrisome behavior. For instance, Romeo's father describes Romeo as having been seen before dawn on many mornings on the west side of the city, near a sycamore tree, "with tears augmenting the fresh morning's dew, / Adding to clouds ore clouds with his deep sighs" (Act 1, Scene 1). Then Lord Montague further describes Romeo as returning home, locking himself in his room, and sleeping all day.
The very first descriptions of Juliet we see in the beginning of the play are given by her parents. Through her parents, we learn that Juliet is very young and also very pretty. In the second scene, when we see Paris ask Lord Capulet for Juliet's hand in marriage, Lord Capulet responds by saying that she is still far too young, knowing nothing about life, and that Paris should wait another two years, at least until Juliet is fourteen. We see this in the lines:
My child is yet a stranger in the world;
She hath not seen the change of fourteen years,
Let two more summers wither in their pride,
Ere we may think her ripe to be a bride. (Act 1, Scene 2)
Likewise, when her mother is preparing Juliet to think of marrying Paris, she refers to Juliet's youth, but emphasizes her beauty when she says, "Thou know'st my daughter's of a pretty age," meaning that she is old enough to blossom, which is very young. Juliet will be fourteen in two years, therefore, Juliet is only 12 years old.
Hence, from the opening descriptions of Romeo and Juliet, we know that Romeo is sorrowing about something and that Juliet is very young and pretty.