Juliet is not yet fourteen when the play begins: according to the Nurse, she will turn fourteen on the eve of Lammas, the English festival of August 1, and Lady Capulet says it is two weeks and some days until Lammas (Act I, scene 3). Romeo's age is never stated in the text.
The first time age comes up is in the conversation between Capulet and Paris (Act I, scene 2). Paris asks for Juliet's hand, and Capulet replies that the girl is too young and has not yet seen the change of fourteen years, and that it would be better to wait two more summers. Paris retorts that younger women than she are happy mothers.
In scene 3 of the same act, the Nurse says she knows the girl's age to the hour and recalls weaning her on the day of the earthquake, eleven years before. Her mother then asks what Juliet thinks of marriage, and she replies that it is an honor she does not yet dream of. The mother says that in Verona, ladies of esteem are already mothers at a younger age, and that she herself was about Juliet's age when she had her.
By the dates the text gives, the rest of the play fits into less than a week: the secret marriage is on the afternoon after the ball; in Act III, scene 4, that night, Paris says it is Monday and Capulet sets the wedding for Thursday; in Act IV, scene 2, he moves it up to Wednesday, and it is on Wednesday morning that Juliet is found dead (Act IV, scene 5). Since on the night of the ball more than two weeks remained before Lammas, she falls in love, marries, and dies before turning fourteen. The text allows this count, but does not make it out loud.