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davmor1973 eNotes educator| Certified Educator

Juliet is 13 years old. We know this because it is mentioned at several points in the play. For example, here is an exchange between Lady Capulet and the nurse:


This is the matter.—Nurse, give leave awhile,
We must talk in secret.—Nurse, come back again.
I have remembered me. Thou’s hear our counsel.
Thou know’st my daughter’s of a pretty age.
Faith, I can tell her age unto an hour.
She’s not fourteen.
I’ll lay fourteen of my teeth—and yet, to my teen be it spoken, I have but four—she is not fourteen. How long is it now to Lammastide?
A fortnight and odd days.
Lammas was an ancient festival that celebrated the annual wheat harvest. There is a clear link here between the ripeness of the harvest and Juliet's being on the cusp of womanhood.
It is interesting to note that in the original poem on which Shakespeare based his play Juliet is actually sixteen. It is not entirely clear what Shakespeare's motive was in making her younger. However, we must never forget that, as well as being a playwright, Shakespeare was an actor manager and man of the theater. At the time that Shakespeare wrote "Romeo and Juliet" the theatrical convention was that the roles of young women would be played by prepubescent boys. So perhaps Shakespeare was thinking about how the character would look on stage.