I don't think there's a set age at which they "should" be in a relationship. The choice to be in a relationship is completely dependent on finding someone with whom you can build a quality relationship. Children shouldn't date until they understand the difference between someone being "hot" and someone being good relationship material. That said, I think under 14 or 15 is too young to date.
It depends on what they would be doing on the date. If like, your 12 year old wants to date, fine. Have a movie date at your house. They think its dating, but you can keep an eye on them. 13-15 was only group dates for me. At 16 I could go on a one on one dates. So I don't think there's an age limit on dating, just modify the things they do on a date to fit the age! :)
13 and up is when you really start thinking about the opposite gender as more than a friend. It is when your about 15 you actually start acting upon the way you feel about a certain boy/girl. Then around 16 your thinking about physical activity and by 18 you may have or may not have already been physical. Notice how 15 when you begin thinking and 16 when you become physical is very close? Well that's when people are immature they aren't sure what to call what they feel. They think it's love, so they get physical with their boy/girlfriend. That is usually not a true statement and they end up breaking up. I think you should have a bit of experience about relationships, and to have that you do need to date a bit early, I'd say 13.