Youngsters have normalized premature grey hair, but why is it so common these days?

The most common reason for this is genetics. If someone in the family has prematurely gray hair, then the teens in the family are more likely to experience gray hair. 

A diet lacking nutrients like vitamin D, vitamin B12, iron, zinc, and copper can lead to premature graying of hair in teens and youngsters. Stress and emotional factors are also things that can contribute to accelerating the graying of hair.

Environmental factors such as pollution and UV exposure can lead to dead melanocytes (cells responsible for hair color). Prolonged damage to the melanocytes can lead to premature graying of the hair. 

Several other lifestyle factors, such as smoking, drinking, drug consumption, and hormonal imbalances, can also contribute to hair graying.