At HumaneTeen young people make a difference for animals and their world. This site is maintained by the youth education division of The Humane Society of the United States (HSUS).
The ASPCA's website contains information about humane education, animal care and animal advocacy.
Just because it's "always been that way" doesn't mean it's right. At peta2 you'll find tools to effect immediate change for animals. This is a teen-oriented website maintained by People for the Ethical Treatment of Animals (PETA).
The Animal Planet site has information about animal-related television programs on Animal Planet, animal videos, and information about all types of animals.
Vegetarian Teens will find great resources at The Vegetarian Resource Group.