Go to an opthalmologist so that you can rule out macular degeneration, glaucoma or cataracts.

I've worn glasses since I was 14 and have never had any problems with viewfinders. One of my lenses is -10 diopters. If you have trouble seeing close (i.e. need reading glasses) you might want to take your glasses off to look closely at the viewfinder. You will definitely need your glasses on to use the magnifier.

I always take them off when I am under a darkcloth.