It is well-documented that testosterone levels decline with age in men.
After the age of 40 years, total testosterone decreases on average -4 ng/dL ( -0.124 nmol/L) per year  or 1.6% per year , and bioavailable testosterone by -2 to 3% per year. 
In older men (over 60 years of age), the average rate of decrement in total testosterone levels has been found to be 110 ng/dL every decade.
However, the relative contributions of changes in health and lifestyle to that decline have not been adequately evaluated. A notable study was set out to investigate this...
The objective of this study was to establish the relative importance of aging, health, and lifestyle in contributing to the testosterone decline in aging men.
A notable study shows that men aged 20–79 who have low testosterone levels below 8.7 nmol/L (250 ng/dL) have a more than two-fold increased risk of mortality from all causes, compared with those with higher serum testosterone levels.
Importantly, this risk is independent of age, waist circumference, smoking habits, high-risk alcohol use, and physical activity.