I don't buy that. The thing is a lot of vegans spend years eating meat and building a strong foundation, which thay can maintain once they become educated vegans.
What I would like to see happen is an experiment where certain babies are placed on a strictly vegan diet (no mother's milk only vegan baby formula), others on a vegetarian diet and others on an omnivore diet. Then, we can watch these babies grow up and see how they turn out.
Although most likely unethical, I think such an experiment will give us a better picture of which dietary preference is the best. The experiment be even better if we used twins, and gave 1 twin a vegan diet and the other an omnivore diet.