Why do I have low testosterone?

“It has been argued that the age-dependent decline in sex-steroids (testosterone/estrogen) and growth hormone production is nature’s way of protecting us from cancer and heart disease, but a far more likely scenario is that once we reach our reproductive capacity, nature begins programming us for death.”  — Roy G. Smith, Ph.D., Director Huffington Center on Aging, former VP of Research at Merck Laboratories

The Short Answer:

As we age our production of testosterone declines by 1-3% per year after age 35.