Whether or not a teenager should be allowed to have a serious relationship depends completely on the teen and who they are involved with. I believe that some older teens are mature enough to be involved in relationships, as long as the other person in the relationship is in the same age group and also of an adequate level of maturity. For centuries people were married in their teens. Marriage later in life is a very recent phenomenon. Parents should also consider the amount of responsibility and maturity their teen has displayed before making this decision.
Teenagers have the bodies of adults, and although their brains are continuing to develop, they should be allowed to have serious relationships like adults do. This allows them to practice good relationship skills and learn to make positive choices while they still have the involvement of their parents to lean on.
Teenagers in serious relationships can lead to both true love as well as severe heartbreak. While the latter is extremely unpleasant, it is a reality that exists all too much in the adult world, and one that most everyone goes through at some point in his or her life. Telling teenagers they cannot have relationships until they are 18 only delays understanding of relationships and heartbreak, but does not prevent heartbreak in itself.
Teenagers should be allowed to explore romance as a natural part of maturation. There is no need, however, for them to have serious relationships. For one, most do not have the dating experience necessary to form romantic attachments that ultimately prove to be healthy serious relationships. Also, there are still many rites in early adulthood that they may initially choose to forgo in the name of a serious relationship but regret later.