Which factor is NOT typically affected by age with respect to drug metabolism?

Muscle function is not typically a direct factor affecting drug metabolism in the context of how age impacts pharmacokinetics. Drug metabolism primarily involves biological processes that occur in the liver (liver function) and through renal excretion (kidney function). As people age, liver size, blood flow, and enzyme activity can decline, leading to altered metabolism of many medications. Similarly, kidney function, which is crucial for the elimination of drugs and their metabolites, can also decline with age, affecting drug clearance.

While muscle function can influence overall health and activity levels, it does not have a direct impact on the metabolic processes that alter how drugs are processed in the body. Absorption rates, while not as directly related to metabolism, can also change with age due to alterations in gastrointestinal function. Therefore, muscle function stands apart as it does not play a significant role in the pharmacological processes affected by aging in the same way that liver and kidney functions do.