How common are vision problems in older adults?

Common. Older adults have the accumulated changes in refraction acquired through the years. To this is added any diseases they might have acquired which might cause vision problems such as diabetes, glaucoma, etc. And finally the additional vision issue related to middle age after age 40 (presbyopia) when near correction is needed.
Depends. On your definition of vision problems. With an aging population, eyecare issues will be common.