Vegetarianism is no longer a counterculture statement of shaggy hippies in Birkenstocks. It's a popular lifestyle choice for many people, including teens, who want to take control of their diets. Some see it as a way to eat healthily; others turn to it because they're animal rights activists who don't like the thought of animals being killed for people to eat. Professional trend-spotter Faith Popcorn predicts a greater number of people turning to vegetarianism in a permanent lifestyle shift. However, for your teen, it's more likely a fad—something she'll experiment with for a time before moving on.
The level of your teen's vegetarianism should dictate your level of concern over whether her body is being well-nourished. If she's kept dairy products in her diet, then she's probably getting all the nutrients she needs. If she's opted for a stricter regimen, talk to your doctor, consult a nutritionist, or investigate a book on vegetarianism such as Diet for a Small Planet. More creative food preparation will be necessary to be certain she gets what she needs.
It's possible to claim to be a vegetarian and subsist on a diet of sugary cereal and corn chips. If your teen has taken to this kind of vegetarian diet, you may want to guide her to more nutritious eating—with or without meat.