>In the story it's an 18 year old, just finished their exams and they're waiting for their results when they find out rather suddenly that they're losing their sight(subject to change to be medically accurate).
You might want to look into Leber's Hereditary Optic Neuropothy (LHON) which causes sudden loss of central vision in one eye, followed by the other relatively quickly (within months). Also the onset is in the age range you are looking for.
$1