A teenager can go blind, slowly?

Blind teens. If born sighted, and in the absence of something obvious like trauma, the conditions that can gradually cause vision loss in the teen years are inherited and acquired retinal dysfunctions (like retinitis pigmentosa), tumors in the eye and cns, uncontrolled diabetes with retinopathy, and severe nutritional deficiencies.
Yes or no. The simple way to answer the above would be yes, however, without knowing the details, it's a matter of guess at this time.