There are different types of testicular cancer. The most common types are teratoma and seminoma. Teratomas usually affect young guys of 15-35 years old. Seminomas usually affect men of 25-55 years old.
The information here is about teratoma, because it’s the commonest type to affect teenagers.
Although we use teratoma (as it’s easier to say), the correct term is Non-Seminomatous Germ Cell Tumours (NSGCTs). They start in sperm-producing cells called germ cells.
Treatment for testicular cancer is really successful and nearly everyone is cured.