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.