Sexual desire is of two categories: Raw sexual desire (also called horniness) and desire in a chosen relationship. Raw sexual desire is innate, biological and is determined by the level of testosterone hormone in blood. This is highest around 17, and stays high up till the age of 24, imperceptibly declining till middle age, thereafter perceptibly. On the other hand, the sexual desire in a relationship depends on the closeness with a partner, and how you are comfortable with a partner. It often happens that the young men who have low desire have normal testosterone levels, but due to interpersonal difficulties they face lower sexual desire. Normally these men respond positively to erotic material and they frequently masturbate when alone, evidence that they have normal hormonal levels.