The guidelines for dating can be found in the For the Strength of Youth booklet (which is official church doctrine) Here's a link to that section: 
http://www.lds.org/library/display/0,4945,30-1-7-4,00.html 
It does not say that a member cannot date a non-member, but rather "Because dating is a preparation for marriage, date only those who have high standards, who respect your standards, and in whose company you can maintain the standards of the gospel of Jesus Christ." 
Of course, some members decide not to date non-members, but that's a personal choice. 
As for the other question here, the same document says "do not date until you are sixteen years old". I would imagine that one that followed that counsel would likely only date someone who was also 16 or older. But that would depend on their attitude: There's the "how bad can I be" attitude (who would), there's the "how good am I supposed to be" attitude (who might), and the "how good can I be" attitude, (who would not). 
At any rate, there are members who take the guidelines and commandments seriously, and there are those who do not. And whether they do or not says a lot about what kind of person they are. You probably want to be dating those who do take it seriously. They're the ones most worth dating.