Total testosterone goes down by about 1-2% per year , starting sometime in your 30s, and bioavailable testosterone decreases by about 2-3% a year. Those numbers can be pretty misleading though. Men approaching middle age tend to exercise a lot less, and eat a lot worse. So nobody really knows 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 to attract a partner (among other reasons, of course).