I think many people are simply unsuited to a vegan diet as there digestive system can't handle for a start (and that is even after three year or so of 'converting'). But I think another overlooked affect of veganism is that most convert after a childhood of eating dairy and eggs and often meat, too. So they already have built their bones and bodies and stored essential minerals etc. Thus it is only after a period of depletion that the symptoms show up. This makes it all the more concerning that much younger children might adopt these diets.