Teenagers will inherit a worse Earth than they see now. If they want clean air when they get older, and a safe place to raise their children, they need to take action now. Teenagers should also be aware of the environment because good habits developed now will help them throughout their lives.
Everyone can be more responsible in this area, especially those in control, those with the power, and those whose decisions and policies really affect the environment more than the average regular consumer. Teens should definitely be aware of how the environment is affected by everything from car fuel to landfills.
Should the be held more responsible, I'm not sure. I do know that they should be raised to know that the CAN be more responsible for the environment. They should be raised to know that they can speak up and out about situations that hurt the environment. Sometimes youth feel as if they have no voice, but they only way they will get voice is if they speak out.
I don't think there is any reason to single out the group of people we call "teenagers" either. Almost everyone, particularly in the United States, could simply decide to consume less, still lead the same quality of life, still have lots of nice stuff and have fun and solve a huge portion of the world's environmental woes. But we all choose not to and so things go on as they are.
Sure teenagers could do more, but so could everyone else.
I don't know that you can point the finger of environmental irresponsibility at any one age group. To me, such responsibility falls on each of us, any gender, nationality, income level or age group. One could also argue that we are all quite environmentally irresponsible, wasteful and over-consumptive.
It does stand to reason that adults that raise their children to value the environment will tend to grow up with those values and pass them on to their own children. In this way, adults have a greater responsibility in my mind.
Are you asking if A) teens should be more responsible for the environment than they currently are or B) if they, rather than adults, are the ones who should be responsible for protecting the environment? If you are asking about the second of these, you can make the argument either way.
Teens should be more responsible than adults because teens are the ones who will be dealing with the consequences of environmental degradation. Adults today will not live to see nearly as many of the consequences as teens will. Therefore, teens should be the ones responsible. In addition, teens are less set in their ways than adults. It is harder to make adults change to be more "green" than it is for teens to simply grow up green. Therefore, it's easier (and makes more sense) for teens to be the responsible ones.
On the other hand, adults are the ones who can have the greater impact. Adults are the ones making the major decisions. Teens really don't even get to decide anything like what kind of car the family buys next, let alone about things like energy policy for the country. Because teens have so little "power" it makes no sense to say teens should be more responsible for taking care of the environment.