The shape of the curve across a lifetime
Male testosterone follows a recognisable arc. A surge during puberty drives the development of secondary sexual characteristics, after which levels plateau at their highest through the twenties and into the thirties. From around age thirty onward there is a gradual decline, commonly cited as roughly one to two percent per year on average, though the exact rate varies between individuals and studies.
This decline is slow and continuous rather than a sudden drop, which is why many men remain within a normal range well into older age. Testosterone also follows a daily rhythm, peaking in the morning, so the time of a blood draw affects the reading.
Why averages hide a lot of variation
Population averages mask wide individual differences. Body weight, sleep, chronic illness, and medications all influence levels, and obesity in particular is associated with lower readings. A value that looks low on a single afternoon test may be normal on a morning retest, which is why patterns matter more than one number.
This is general educational information, not medical advice. Interpreting testosterone levels and any symptoms should be done with a qualified clinician.