I did not start dating until the age of 18 and it's amazing how much that benefited me throughout my womanhood and during my adult relationships. Therefore, I'm a firm believer that during middle school and high school, girls (and guys) should be focused on preparing to tackle adulthood, setting up the correct framework for life as an adult (college), and enjoying what's left of their youth before they get a taste of the real world. Exposing your heart to temporary relationships where it my get broken or permanently scarred is simply not a wise, productive use of time during those precious years. It's so much better to just wait.
The older and more mature they are, the better. Most teenage relationships don't last, because they simply don't have a clue what they're doing (no offense to anyone; some people actually do know). Teenage relationships are for the initial experience, and nothing more. They learn a little bit about romance and dating, and that carries them further to find the right person.
I voted for 16, but that is the minimum age that I think people should start dating. It shouldn't be anything less. 16+ seems like a decent age.