You ask a very interesting question that is a hot topic in cognitive psychology and neuroscience. Actually, just last week a fellow PhD student presented some of her work on this topic related to false memory. I will explain the idea of false memory as it is studied in psychology and then give you a speculative answer to your wonderful question.

The main point that the concept of a false memory makes is that a memory is not just a simple recall of the past. Rather, it is a constructive generation that is dependent on the immediate cues as well as our recollection of the past. For example, some of the original studies performed by Elizabeth Loftus involved showing people pictures of a sports car passing a stop sign. Loftus then asked people questions like, "Was the Datsun going fast when it approached the yield sign?" As you can see, the experimenter has introduced misinformation because the participant would now believe that there was yield sign instead of a stop sign.

There are a few factors that affect eyewitness testimony. One has to do with how calm or aroused a person is when the crime or event took place. Less arousal (more calmness) leads to eyewitness testimony that is more accurate. The content of the event that is being remembered might also make people more or less aroused. For example, seeing a gun makes you more aroused and less calm. Another factors is whether there is social pressure to think a certain way. This comes up in eyewitness testimony when people on the stand in court are pressured by lawyers or other societal norms to think a certain way. Finally, receiving positive feedback has been shown to lead to more false memories. For example, if when people identify a suspected criminal in a police lineup, an officer replies with a simple "OK", then it is considered as positive feedback and it is enough to create a false memory.

Based on the factors I have told you that lead to false memories and less accurate eyewitness testimony, teenagers are probably less reliable eyewitnesses. The question is not whether they have better memories, but that they are more prone to being aroused and victims of social pressures.

As a side note, I have spoken to several professors in our department that study memory and they agree that no one has experimentally studied age effects on eyewitness testimony comparing teenagers and normal functioning adults. If you're interested, you should contact Elizabeth Loftus as she is at UC Irvine.