Simple question. 1 adolescent brains have yet to fully developed. they are prone to impulsive behaviors. 2 teenagers are generally naive.

Because when you are in your late twenties, you aren't just searching for a boyfriend/girlfriend but your future partner in life... and since most people you meet are not going to be your future spouse (which is logical), you may feel it is hard to find "The One". (It really is - there are several million people who could qualify for the position, so finding that person in a short time requires huge luck.) The other factor which may hinder people in finding their partner may be that they have problems they should come to terms with first, but this is another topic, and a long one.