This document discusses several biological markers of aging, including theories of aging like the free radical theory. It outlines how hormones like testosterone, estrogen, progesterone, cortisol, DHEA, human growth hormone, and melatonin naturally decline with age and how this affects physiological functions. Graphs show patterns of these hormones over the lifespan. Abnormal levels of hormones like cortisol and melatonin are associated with various health conditions. Declines in hormones like growth hormone, which regulates aging processes, are also examined. The final section discusses calorie restriction and related compounds that can modulate biological markers of aging by activating pathways like AMPK and sirtuins.