How old was Lady Capulet when Juliet was born?

It appears that Lady Capulet was quite young when Juliet was born.  When she is introducing the idea of marriage to Juliet before the masque in Act I Scene 3, she mentions that she was already a mother by Juliet's age, which is not quite 14:

[quote]

Lady Capulet.
Well, think of marriage now: younger than you,
Here in Verona, ladies of esteem,
Are made already mothers: by my count
I was your mother much upon these years
That you are now a maid. Thus, then, in brief;--
The valiant Paris seeks you for his love.