Cane you name some ways a teenager can go blind, slowly?

Why? This is a concerning question. You can google that question and get answers. If it is a real concern then you are best served by seeing an opthamologist- an eye specialist.
Vision loss as teen. Asymptomatic (no pain, injury, redness) slow vision loss is likely to derive from either inherited retinal disease like retinitis pigmentosa or from central nervous system disease such as tumors affecting the visual pathways (pituitary, occipital lobe). An ophthalmologist should be consulted to sort these out.