Teens are very impressionable, however it's not movies, music, or video games that does it, it's other teens. Mags, you're right on the money as to why. "All reasoning left you" is pretty apt. When you did "stuff" you weren't reasoning, it was instinct.
All other things being equal, (i.e. not suffering some major catastrophe like war, Katrina, debilitating disease or accident, or famine etc.) being a teenager is probably the toughest/worst and most stressful time in a humans life.
Essentially, you've got a body that's 100% geared up for mating, and all the instinctual and social behavior that pushes you to do it, and more intensely than it ever will again, but our cultural and societal norms are telling you that you shouldn't be having children for another five to ten years.
That's a huge discrepancy.
When all this comes to a head, acceptance is everything to a teen. What we call "peer pressure" is really just the overwhelming urge to be accepted by your peers. Humans, just like all group mammals, feel this urge to some degree or another our entire lives, as it's a survival trait, but it's never more intense than it is right after puberty. While we're much more independent than herd animals, we're not solitary hunters like sharks or crocodiles either.
Because of the new mating drive, puberty suddenly has your entire genetic future screaming at you subliminally to make sure that you are accepted by your "tribe" and to start searching for acceptable mates. If you fail at this task, are excluded, ostracized, or not dating (i.e. practice or "trial mating"), your biological side sets up warnings that you're at risk of becoming a genetic dead-end. From a purely physical standpoint, that's the entire purpose of your existence along with all other life on this planet.
It's just like the apes in a Jane Goodal documentary. The poor chimp that suddenly gets beat up and cast out of the troop to fend for itself in the jungle either has to find it's way back into the group, or find a new one. Otherwise, that chimp might as well be among the walking dead, and instinctively, they know it. It plays out differently for us, filtered through culture and societal custom, but it's there, none the less. We let things like the Internet, thousand-channel cable TV, forks, cars, and nuclear weapons distract us from that reality to our biological sides, but it's always there, just beneath the surface.
There's other contributing reasons to be sure, poor judgment, bad upbringing, "testing limits", and simple evil intent, but underlying every dumb, dangerous, or criminal thing that teens and young adults do comes of the need for acceptance by their group lest they get dealt out of the game of life and not pass on their genes.
The other, even more dangerous side, are teens who feel as though they arlready are ostricized and have nothing to lose. School shooters are probably the prime example of this. Most all of them feel as though they've been excluded by the larger group, they are in that "walking dead" category, and turn both suicidal and violent. The animal kingdom is full of examples of animals that have turned "rouge", such as lions, elephants, and certain monkeys. They've been expelled from the group, and it's common for these animals to be more violent and aggressive.
No one should mistake this as apologizing or dismissing such behavior. We all have our moral and intelectual side that should control these instincts. The fact that these urges and instincts exist, and are so powerful is not an excuse, because as humans our intelects are powerful too, which is the point of being human.