Premature ejaculation in 21-year-old boys may be caused by psychological factors, or it may be caused by certain pathological factors, such as over-sensitive glans, prostatitis, seminal vesiculitis and so on. 1. Psychological factors: if the patient is in a highly stressed mental state for a long time, it may lead to premature ejaculation, in addition, the relationship with the sexual partner, emotional problems can also cause premature ejaculation. 2. Pathological factors: (1) glans is too sensitive: 21-year-old boys if the glans is too sensitive, there can be men in the couple life when the situation of early ejaculation. (2) prostatitis: prostatitis is caused by pathogenic bacterial infection or certain non-infectious factors, patients may appear pelvic pain and discomfort, urinary abnormalities and other manifestations, may be accompanied by varying degrees of sexual dysfunction, premature ejaculation, spermatorrhea, ejaculatory pain and other uncomfortable symptoms. (3) Vesiculitis: Vesiculitis is a common infectious disease of the male reproductive system, as long as the inflammation caused by the invasion of pathogenic bacteria, triggering the symptoms of bloody semen and ejaculatory pain. Sexual dysfunction can also occur in seminal vesiculitis, which is manifested as decreased libido, premature ejaculation, erectile dysfunction and so on. In addition to this, there may be other causes of premature ejaculation in 21-year-old boys, and it is recommended to actively seek medical attention under the guidance of a professional physician.