I believe teens under 18 can have serious relationships if they are very committed. I know a lot of teens are mature and exceed the expectations of what others think of the maturity level of teens these days. I think if you are in a relationship and you want the relationship to last, you have to be committed and give it your best. But you should always get out of the relationship if you feel it's not working out.
Personally, I believe you have to be an adult (i.E. 18 years of age) to be in a serious relationship. That isn't to say younger couples can't care deeply for each other, but going out to dinner once a week or sneaking them over when your parents are at work doesn't constitute a mature, committed relationship. I think a couple can meet young and stay together, in which case their relationship develops into something serious as they get older- but two sixteen-year-olds can't be in a serious relationship in my opinion.
Yes, teens under 18 can have serious relationships, because teens have feelings, just like adults do. However, there are not many teens that are mature enough for a relationship like that. In most cases, the relationship will fall apart and will not last. For the few teens that understand the necessity of maturity, they can have successful serious relationships.
I realize every teenager in a relationship thinks it is serious. However, in reality they are usually not mature enough to understand and experience everything that goes with actually being in a serious relationship. Sometimes it is possible but the majority of people under 18 are not able to commit to a serious relationship.