Is it possible to get open angle glaucoma when you're 13?

Yes but unlikely: Yes, but very rare. Usually open-angle glaucoma is a disease of aging. Most often, adolescent glaucoma comes from trauma or congenital defects.

Yes: This is juvenile onset open angle glaucoma which is very rare and typically inherited. Most of the time there is a problem with the drain of the eye not functioning well and thus the pressure is elevated leading to glaucoma.

Yes: Unusual but possible and congenital in nature.