The harm of 12-year-old masturbation is mainly caused by bacterial infection, sexual dysfunction and so on. 1. Bacterial infection: due to the stimulation of masturbation, caused by the prostate persistent congestion and swelling, easy to cause prostatitis, may also lead to bacteria into the urethra, causing urethritis, frequent urination, urinary urgency, urinary pain and other uncomfortable symptoms. 2. Sexual dysfunction: If the number of times of masturbation is too frequent, it may also lead to excessive excitation of sexual nerves, which will lead to premature ejaculation and erectile dysfunction when you have sex in adulthood, which will directly affect the quality of sexual life. It is recommended to avoid masturbation, actively develop good habits, ensure balanced nutrition, keep the pubic area clean and hygienic.