Testosterone levels do not naturally decline
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.
Citations:
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.