A major issue with correcting genes in adults is the the large number of cells that needs to be corrected. Let's say you have a muscle disease, hitting every single muscle cell in you body to correct an genetic disorder is very complicated. As of right now, this is easier with blood cells. You can isolate hematopoietic stem cells, that you can edit and then reintroduce in a patient. Those cell will repopulate the immune system.

Every disease is different. For many diseases it might be enough to correct 10% of the cells for a patient to be able to live with the disorder. Some of those examples already exist today. Some more complicated genetic disorders we might never be able to correct (at least in our life time).