From the article:
However, urologists are seeing a growing number of younger men with concerns related to testosterone deficiency – often with less-specific symptoms such as low energy and fatigue.
Diagnosing low testosterone in young men poses other challenges as well. The standard cutoff point for low testosterone is 300 nanograms per deciliter (300 ng/dL). However, that threshold is based on testosterone studies of older men, and overlooks the normal age-related decline in testosterone levels.
To develop a set of age-specific cutoff points, Dr. Zhu and colleagues from the University of Michigan Department of Urology analyzed data on nearly 1,500 men, aged 20 to 44 years
[…]
As expected, the testosterone levels decreased at older ages. Age specific cutoff points for low testosterone ranged from 409 ng/dL at age 20 to 24 years to 350 ng/dL at age 40 to 44 years – substantially higher than the standard cutoff point. Each one-year increase in age was associated with a 4.3 ng/dL decrease in testosterone level.