I have always had poor vision, and have worn glasses. Lately, my eyes have been taking longer to adjust, and I can't focus them. What can cause this?

See below. In older people, it's called presbyopia: the lenses don't change shape for near & far as quickly as they once did. Not a likely cause at your age. Are u diabetic? Have u been tested for diabetes? When was your last thorough eye exam? See a well-trained eye doctor as well as your fp to be checked out for that possibility.
Check a few things. Begin with the eye exam. There are a variety of tests that can be done to assess how well you adjust from distance to near. Next, get a physical and be tested for diabetes. Changes in blood sugar can create symptoms like this. Make sure you are not being exposed to any new drops or on medications that could cause this effect. Again, complete medical exam.