Ridiculous statement. Studies have demonstrated time and time again that a well-balanced plant-based diet is appropriate at all stages of human developpement:
"Nowadays, plant-based eating is recognized as not only nutritionally sufficient but also as a way to reduce the risk for many chronic illnesses." - Harvard Medical School
" [...] appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases." - The American Dietetic Association
When it comes to facts, I'll favour serious scientific meta analysis over some anecdotal claims, thank you very much.