If I get no random erections during the day at age 21 is that a sign of low testosterone?

Not necessarily. The occurrence of random erections can vary significantly among individuals and can be influenced by factors such as stress, anxiety, sleep quality, and overall health, in addition to testosterone levels. While a lack of random erections can sometimes be associated with low testosterone, it's important to consider other factors and consult a healthcare professional for a comprehensive evaluation if you're concerned.