The amount of testosterone in the bloodstream increases during adolescence and peaks in a man’s twenties. By age 30 or 35, testosterone levels decrease by about 1% each year. This gradual drop is a normal part of aging. By age 80, half of all men have low testosterone, according to the Urology Care Foundation. But testosterone levels can fluctuate daily, and even change from hour to hour. They are highest in the morning, which is why doctors run blood tests for the hormone between 7 and 10 a.m.