Middle-aged[1] and older[2] men see their total testosterone levels decrease by 0.4% to 1.6% per year, many of whom had lower-than-average levels even in their 30s.[3] Bioavailable testosterone decreases by about 2–3% a year.[1]
Those numbers can be pretty misleading, though. Men approaching middle age tend to exercise a lot less and eat a lot worse. So it’s hard to say what a “natural” decline in testosterone looks like on a population-wide basis.
It’s kind of like saying muscle mass decreases 1–2% a year once you hit middle age. That decrease can have a lot to do with more time spent on work and family and less time spent trying to get ripped.
References
- ^Feldman HA, Longcope C, Derby CA, Johannes CB, Araujo AB, Coviello AD, Bremner WJ, McKinlay JBAge trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts male aging studyJ Clin Endocrinol Metab.(2002 Feb)
- ^Wu FC, Tajar A, Pye SR, Silman AJ, Finn JD, O'Neill TW, Bartfai G, Casanueva F, Forti G, Giwercman A, Huhtaniemi IT, Kula K, Punab M, Boonen S, Vanderschueren D, European Male Aging Study GroupHypothalamic-pituitary-testicular axis disruptions in older men are differentially linked to age and modifiable risk factors: the European Male Aging StudyJ Clin Endocrinol Metab.(2008 Jul)
- ^Handelsman DJ, Yeap B, Flicker L, Martin S, Wittert GA, Ly LPAge-specific population centiles for androgen status in menEur J Endocrinol.(2015 Dec)