Well, according to a new piece in the UK newspaper Daily Express, if you are a man older than 19, then your testosterone levels are already dropping.
It is important to note that even though the decline starts after early adulthood, and so the Daily Express is not off the mark there, but levels in the twenties are typically still okay for general functioning. In later decades of course, the lower levels are connected to various signs of male ageing.
The piece cites a conversation with Bertie Stringer, head of nutrition at men's hormonal health company, Dynamic Nutrition Academy, to come up with a list of telltale symptoms that indicate falling testosterone levels in men.
The list they have come up includes the usual suspects such as reduced sex drive, erection challenges (loss of morning erections specifically), enlarged breast tissues, loss of muscle mass, increase in anxiety and stress and other such. A better way for self evaluation is of course through the clinically studied ageing male symptoms (AMS) questionnaire, which you can go through right here. Takes less than four minutes, and apart from knowing your scores you can also index your experience to that of other men who have taken the assessment.The AMS self assessment
The piece also throws some interesting reference points around the linkage of declining testosterone levels with obesity, stressful jobs and unhealthy lifestyles. According to research cited (although it would be better if the Daily Express took the effort to name the publication where said research was published), testosterone levels are declining in general across the board, with the drop being 20% over the last 20 years.