Humans are actually omnivores, not carnivores. Different species have different dietary requirements to each other, dogs for example while primarily carnivorous, are not obligate carnivores. They CAN actually be vegetarian. A cat on the other had IS an obligate carnivore, they must have meat in their diet. The reason being that dogs only require 10 essential amino acids in their diet while cats require 11, the 11th being taurine which is primarily only found in animal tissue. Cats do not have the ability to synthesis taurine and without it in their diet are subject to a great deal of health problems. Humans, just like dogs, are not obligate carnivores.

The anatomy of omnivores and carnivores are similar (as opposed to herbivorous animals both hind and fore gut fermenting), yet they also differ. Omnivorous teeth are a cross between herbivore and carnivore, but we do not have sharp edged or fanged teeth like that of a cat. Even looking at the teeth of a dog and cat, while similar initially, they are quite different.

Children have more trouble being vegetarians than adults because they do have different requirements to adults. Children are not able to create taurine the way adults can, so they are subject to more risk.

People who are uneducated before becoming vegetarian can also be at risk, because they may not know the things they are lacking (most people will focus on iron but there is also as said things like Vit B12). There is also the misconceptions that you will get all your iron needs from things like spinach and broccoli. The iron in vegetables is different to the iron in meat (heme and non-heme) and is also harder for the body to process so you do need to eat quite a lot more, there is also the fact that other vitamins and minerals can also have an affect on the iron absorption. Calcium binds non-heme iron meaning even less is processed, whereas Vitamin C helps aid the body's absorption.

As I said before I have been a vego for 10 years. I have NEVER had a single health issue caused by my diet, and I've never supplemented my diet with pills. I've even had blood tests done. Not every vegetarian is subject to problems, having said that not every vegetarian DOESN'T suffer issues. Every human body is different.