Also I know "so many" vegetarians and vegans who aren't anemic. It's really not a foregone conclusion. I also know people who eat meat that are anemic. Many Americans probably don't have a particularly well planned diet, especially children and early adolescent girls (who are also generally menstruating for the first time and having more freedom in their food choices at lunch, and may not want to eat with peers). It doesn't necessarily have to do with being vegetarian, and it requires really only marginal planning to get the correct nutrients.
The girls who eat meat are the ones who will get strong? I'm surprised anyone even knows what a bunch of teenage girls could possibly be eating. I guess that's why my vegan self is much stronger than the average female my age. I also never get sick and I donate blood regularly so I know I'm not anemic (in fact the nurse often comments on how high my iron counts are), although the plural of anecdote is not data and all that. Everyone should educate themselves on basic adequate nutrition. I feel really strongly there is a severe lack of logic based nutritional thought in our society and the milk industry just comes in with "milk will solve all your problems." Really? I'm not saying veganism is inherently healthy and that's not really my motivation either so that would be fairly hypocritical (I also don't believe it's "unnatural" for humans to drink cow's milk, but it's just silly the way we're conditioned to believe it's perfect and necessary), but some of the assumptions I see on a daily basis are really ridiculous. It's not that hard to get protein, iron, vitamin D, or calcium as a vegan. It requires a little awareness of what you're eating, and regular eating. This is also a true story with any dietary lifestyle.
Eating a whole bunch of meat and dairy is probably rarely the answer either. At some level saturated fats and a lack of vitamin C can impede absorption of iron, calcium, etc as well as cause other health problems. You know what the problem with some (perhaps even "a lot" would not be hyperbole in thise case) teenage girls is? They just don't eat. They skip meals. Throw their lunch away. Drink diet coke (I'll admit, diet soda is one of my unhealthy choices, but that stuff is pretty much helping no one). Anyone who's ever been a teenage girl probably knows that. People don't distinguish between methodical eating disorders (more rare) and the very common disordered eating patterns. There's often little awareness of regular, sufficient eating no matter what is eaten, and a lot of fears over whether it's too much, looks bad, don't want to eat in front of the boys, etc. I have no complex algorithm for eating and I'm a perfectly healthy vegan. I eat a variety of foods and I eat regular meals that contain basic food groups. A lot of food is fortified and I don't have absorption problems. If I did or if I somehow develop them, I know what the most efficient supplements are. That's about it. But it requires a willingness to eat regularly in the first place. Although I've never spiralled too far down, I have to admit there are times when I haven't had that in the first place.