Many people tell me "young people aren't mature enough for a serious relationship", but although maturity can often develop with age, I know teenagers who are more mature than middle aged people. The age at which people devote themselves to a "serious relationship" is and should be, a very personal issue.
This being said I don't think people should be entering a "serious relationship" until they are atleast a teenager. I say this because relationships can often be stressful, and take effort, whereas teens should be out enjoying themselves with friends. You'll have the rest of your life for relationships.
In a relationship you have to be aware of what the other partner wants out of the relationship. If your partner wants something different than you out of the relationship, then when you confess your love to them you could be shot down. This is where being younger could affect you. When you have little life experience you are less well equipped on how to handle rejection. This could be taken as a reason for why you shouldn't have a "serious relationship" too early on in life. On the other hand it could be taken as life experience, helping you to become a more whole person and have experience under your belt on how to handle relationships, rejection, and compromise.
To me this only emphasises the fact that it's not fair to pigeon box young people as immature for relationships, and that everybody should be taken on a case by case basis.