This peer-reviewed Journal of Clinical Endocrinology and Metabolism study analyzed 1,532 serum samples from 835 American men.

The researchers compared men of the same age across calendar years, covering 1987 to 2004, to separate aging effects from time-period effects.

Testosterone showed an additional age-independent decline of about 1.2% per year, supporting a multi-decade downward population trend.

This JCEM paper analyzed 5,350 serum samples from Danish men across four population surveys from 1982-1983, 1986-1987, 1991-1992, and 1999-2001.

The model separated age, birth cohort, and calendar period effects.

Both total testosterone and calculated free testosterone showed significant downward secular trends, indicating lower levels in later survey periods at similar ages.

This European Journal of Endocrinology study used 3,271 men from Finnish population surveys in 1972, 1977, and 2002.

It evaluated age and cohort effects using testosterone, SHBG, and gonadotropin measurements.

Men in later cohorts had lower testosterone at comparable ages, and the paper also reports age gradients from about 25.3 nmol/L in men 25-29 to 16.9 nmol/L in men 70-74.

This population study in European Journal of Endocrinology compared random samples of adults in Gothenburg, Sweden, measured in 1995 and 2008.

The design matched similar age groups 13 years apart and used fasting hormone measurements.

Men in 2008 had lower serum free testosterone than men of similar age in 1995, and the drop appeared clearly across age groups.

This Reproductive Biology and Endocrinology paper analyzed all incident total-testosterone tests in male members of a large Israeli health provider.

The dataset covered 102,334 men and a 13-year period from January 2006 through March 2019.

The study found a clear age-independent decline across most age groups over time.

This International Urology and Nephrology study assessed 9,054 Brazilian men aged 19 to 84 over a 14-year period.

The analysis adjusted for age and examined yearly change in measured testosterone.

The paper estimated an age-independent mean decline of 10.68 ng/dL per year (around 1.6% yearly), with steeper yearly declines in younger and older subgroups.

This review in Current Opinion in Endocrinology, Diabetes and Obesity synthesized longitudinal studies on testosterone trajectories in older men.

It focused on separating normal age-related decline from additional calendar-time effects.

The article concluded that the secular component may be as large as about 1% per calendar year beyond cross-sectional age trends.

This JCEM longitudinal cohort study followed healthy men in the Baltimore Longitudinal Study of Aging.

The investigators tracked repeated hormone measures over time to quantify within-person decline rates.

Total testosterone declined by about 1.6% per year, while free and bioavailable testosterone declined faster at roughly 2% to 3% per year.

This peer-reviewed Urology study analyzed nationally representative U.S. survey data for 4,045 males aged 15 to 39 years.

Researchers compared testosterone levels across NHANES cycles from 1999 through 2016 and adjusted for factors including body mass index.

The reported trend still showed lower testosterone in later survey years, supporting a time-related decline signal in younger men.