I'd amend this to say "Dating as an adult," since I believe that teenagers should date, if only to get an idea of how NOT ready they are for marriage. However, I don't believe in anything more than casual dating when you're under 18. You're simply not old enough/mature enough to make a life-long decision at that point.
Marriage. Some need to remember what it is all about. You don't give up when times get tough. When you choose to marry someone you are choosing to spend the rest of your life with them through the good and the bad, the easy and the hard times. You stand by that choice, and that person, no matter how hard it is, every single day.