See Below. It is unusual for young people to have cataracts, however, very possible. There are many reasons someone can have a cataract at age 17. I suggest asking your ophthalmologist what might the cause of your specific cataracts, but causes in young people can include trauma, uncontrolled diabetes, medical steroid use, genetic and many others.
Can be genetic. Cataracts (a white lens change): most often due to/made worse by (most common first): aging, genetics predisposition, excessive sun exposure, smoking, diabetes, steroid medication, previous trauma, inflammation, infection in the eye causing inflammation; previous intraocular eye surgery; other genetic conditions; can be congenital; more info: email@example.com.
Early cataracts. Some people can develop cataracts at an early age. It can just happen, although one known risk is blunt trauma.
Congenital. Congenital cataracts are seen at birth, and this is likely what you have. They can have a variable effect on vision.