It is important for men to produce the hormone testosterone and to have a normal testosterone level. This hormone helps with the development of their male characteristics such as their muscle strength, body and facial hair, and their deep voice. Male testosterone is also essential because men need it to be able to produce sperm and for the development of their penis and testes. As men age, it is normal to have a decrease in their testosterone level which may lead to erectile dysfunction and a reduced desire for sex. It is common among young men to produce the most testosterone at the age of 17; they produce it in their testicles.