Wow, thanks for posting this... I've taught the play for three years now, and I was never aware of that historical tidbit. When I think about the fact that Juliet was young to get married even by the standards of Shakespeare's time, for me it heightens the tragedy of the story even more. When I first read Romeo and Juliet in high school, I understood the foolishness of the heroes and sort of thought "how idiotic are THEY?" But once I returned to it as an adult, my feelings softened... especially when I see my students, who are around the same age. Now, when I read it, my reaction is more like: "Gosh, who is looking out for these lost, angry kids?"
To now know that both the arranged and spontaneous marriages of Juliet were even more needless and rushed than I previously believes increases my sense of injustice done to these young characters. Juliet was a pawn in a business transaction (her father's agreement with Paris), and Romeo was a puppet for his trusted Friar Lawrence's desire to bring peace to Verona.
Romeo and Juliet (not to mention Tybalt, Bevolio, et. al.) really are "just kids." That really seals the tragedy for me.