Final answer:
The rate of drug metabolism in children generally reaches adult levels by the age of 12, but specific enzyme systems can mature as early as 1 year old. The exact age can vary greatly due to factors such as genetics and cognitive development which continues into the early 20s.
Step-by-step explanation:
The age at which the rate of drug metabolism in children reaches that of an adult is quite variable, but in general, children's liver enzyme systems become fully mature by about the age of 1 year, with some specific pathways reaching adult levels of activity around the age of 12 years.
It's important to recognize that pharmacokinetics—the branch of pharmacology concerned with the movement of drugs within the body—affects the absorption, distribution, metabolism, and excretion of drugs, and is influenced by various factors including age, gender, genetic characteristics, and food. During adolescence, cognitive development, which includes the capacity for insight and judgment, continues well into the early 20s, suggesting that there might be continued maturation of metabolic functions through these years. The complexities of drug development and pharmacogenomics further underscore the variability of drug metabolism rates among individuals, as genetic polymorphisms can significantly influence the activity of drug-metabolizing enzymes such as the cytochrome P450 family.