The harm begins immediately at the first, and then only intensifies.
All these "diets" are pathological deviations from the biological norm, which cannot lead to anything good.
Our ancestors ate more meat (in percentage) than cave bears and saber-toothed tigers.
It was eating meat and cooking food (in fact, preliminary external digestion, almost like spiders) that allowed humans to grow a brain of such size, otherwise it is impossible to feed the nerve cells.
That is why vegans quickly become stupid and plunge into psychosis.
In short, nothing like this happens at any of these stages. Harm is caused only if a person stops reasonably planning their diet and begins to eat only pasta with ketchup or salad (similar punctures can happen in non-vegetarians).
I have two sons, 25 and 22 years old. It's been 3 years since they became vegetarians. The theoretical basis is different for everyone. The younger one is inclined to Buddhist teachings, the older one is an atheist (just "sorry for the bird"). The needs for renewing the amount of energy through food in our latitudes play a role, of course. And my guys, with the initial mood to eat only vegetables and fruits, periodically break down hard on dairy products. Because otherwise you will not get warm. I forgot to say, we are from St. Petersburg. So climatic conditions also mean a lot in this matter.
There is such a thing as bio-individuality. This means that I feel good when I eat fish, my girlfriend has a great life as a raw foodist, and my friend is great to be a vegetarian. The main thing is to understand which way of eating gives the most energy and vitality. This can be determined by removing for a while different categories of products (dairy, flour, meat), and then entering them back and looking at the result. With vegetarian, vegan and raw food diets, it is very important that there is protein in the diet and that there is no excess of sweets, then everything will be fine.