Testosterone is an androgen found in the body. It controls libido, bone health, muscle development, and immune function. Natural testosterone level is at its peak during adolescence until early adulthood.
As you get older, the testosterone level starts to decline gradually. More often than not, the decline starts by the age of 30 to early 40s. Older men should determine if their low testosterone level is due to normal aging or as a symptom of a disease.