Proteins should preferably be added no earlier than from the age of 16 and you can first try to achieve a natural increase in muscle mass, and then add proteins, which by the way are not always absorbed and work, half of the brands tested by the Russian Federation seemed to be dummies, may have been a fake, or maybe just from low-quality raw materials, tried a lot of proteins from proteins and in different price segments, returned to Optimum Nutrition, I take it from the official https://bioeffect.com.ua/, they always have it, expiration dates are in order, clean working composition without side effects, it is well absorbed and you can see from the result that it is working)).
Well, exactly from passport age and sports nutrition is not pampering, an excess of amino acids is fraught with oncology and diseases associated with metabolism, plus there will be no result without physical activity.
It depends on what and what kind of load and goals, carnitine is possible from the age of 15, and boosters and hydrolysates of aminoproteins not earlier than 18 years old, but in general everything is individual and in consultation with a trainer.
I think that at 16, with a properly balanced diet and physical activity, muscles develop with age. But if you are already 20 and you are like a dick, then it's time to think about sports nutrition. I'm a girl and I eat protein bars after training. I really like the SportExpert line, they have a Protein Bar, the body becomes more prominent and their quality is excellent, still GMP certified.
Better from 18 and only with the advice of competent doctors. Not those in state hospitals, for which the word "powder" in any form is equivalent to drugs and has only a detrimental effect.
Sports nutrition, what is sold in special stores legally, is as dangerous / safe as products from a grocery store, so if its dosages are within adequate limits, then there are no special restrictions. You can look at the composition of baby food and you will find the same protein there as on the shelves of the sports store. I would say that before the age of 16 there is generally no point in taking sports food, but in theory, no problems should arise except financial ones.