Testosterone levels in adult males range from 270 to 1070 nanograms per deciliter with a decrease of 1 percent per year after age 30, according to Healthline. While low testosterone is a normal part of aging, it can be a sign of numerous health conditions, including cancer.
Testosterone levels in men age 40 can be up to 10 percent lower than those of the average adult male aged 20 to 30. Some things that cause lower levels include depression, increased weight with little physical activity and certain medications, as WebMD explains. Low testosterone can be changed by increasing activity level, losing weight and avoiding glucocorticoid medications such as prednisone or opioid pain medications.