Final answer:
Histidine is considered essential for infants and children to support their growth but can be synthesized by adults.
Step-by-step explanation:
The essential amino acid that is essential for young people but not for adults is histidine. While adults can synthesize histidine sufficiently, infants and children require it from their diet to support their growth and increased protein synthesis.
Proteins, which are crucial for life, comprise 20 standard amino acids, 9 of which are essential as they cannot be synthesized by the human body and thus must be obtained through the diet. These essential amino acids include methionine, valine, isoleucine, leucine, phenylalanine, threonine, tryptophan, lysine, and histidine.