The Oxford edition I have does say that. But when we read the play in school the text seemed a little garbled. Probably because we had an awful cheap edition lol
Our teacher said she was 12. Not sure if she was joking or not though
Lady Capulet. This is the matter:—Nurse, give leave awhile,
We must talk in secret:—nurse, come back again;
I have remember'd me, thou's hear our counsel.
Thou know'st my daughter's of a pretty age.
Nurse. Faith, I can tell her age unto an hour.
Lady Capulet. She's not fourteen.
Nurse. I'll lay fourteen of my teeth,—
And yet, to my teeth be it spoken, I have but four—
She is not fourteen. How long is it now
To Lammas-tide?
Lady Capulet. A fortnight and odd days.
Nurse. Even or odd, of all days in the year,
Come Lammas-eve at night shall she be fourteen.
Susan and she—God rest all Christian souls!—
Were of an age: well, Susan is with God;
She was too good for me: but, as I said,
On Lammas-eve at night shall she be fourteen;
That shall she, marry; I remember it well.
Therefore, in "a fortnight and odd days" (or a little over 2 weeks) Juliet will turn 14. She is, therefore, 13.