Researchers searching for reference ranges for testosterone levels among age groups discovered that there may not be a rationale for any decline in testosterone due to age alone.
Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe The Journal of Clinical Endocrinology & Metabolism, Volume 102, Issue 4, 1 April 2017, Pages 1161–1173, DOI:10.1210/jc.2016-2935
Serum testosterone, dihydrotestosterone and estradiol concentrations in older men self-reporting very good health: the healthy man study. Clin Endocrinol (Oxf). 2012 Nov;77(5):755-63. doi: 10.1111/j.1365-2265.2012.04432.x.