This is absolutely a great thing to do with your old glasses.
My mom's been working with the Lions' Club for years, conducting eye exams and fitting glasses for local children from families who otherwise could not afford them.
What they do with the donated glasses, though, is use them in their mission trips to third world countries. Mom's traveled to Haiti on some of these trips, where an amazing number of people with no access to any sort of regular medical care are screened and fitted with the donated glasses. Doctors also perform many, many cataract and other eye surgeries on these trips.
When glasses are donated, volunteers check them for damage, repair what they can, read the scrips, and label the glasses. Then, when they do the eye exams on their trips, they take the supplies of donated glasses along. As each patient is examined, the Lions volunteers check the donated glasses to get the closest fit they can to each patient. The glasses are fitted on the spot, and many of these people are able to see well for the first time ever when they leave these traveling clinics.