Final answer:
Testicular cancer is the most common tumor in men under 20 years old, with early detection often leading to successful surgical treatment.
Step-by-step explanation:
The most common tumor in men under 20 is testicular cancer. This type of cancer typically occurs when cells in the testes grow out of control, resulting in a lump known as a tumor. Despite prostate cancer being the most common type of cancer in men generally, it tends to affect older individuals, whereas testicular cancer is more prevalent in younger males, particularly those aged 15 to 35 years. Testicular cancer is often accompanied by signs such as a lump or swelling in one testis, fluid accumulation in the scrotum, and pain or tenderness. Fortunately, if detected early, testicular cancer can usually be cured with surgical procedures.