All of the above plus muscle atrophy that makes stuff like IT band issues feel like knee pain. I'm in phenomenal shape for a 40+ year old and I have to spend a significant amount of gym time doing yoga and correctional lifting instead of body building style lifting.

As other people have said it's somewhat all of the above, though the biggest and most visible on xray is the narrowing of the joint space from deterioration of the cartilage. This causes arthritis, and also calcification of the joint which is also all worsened with age and varying bone density which can change based on things like ancestry, gender, habits (drug use/smoking) or medical conditions.

Edit: I also forgot to mention that occupation can make a big difference here. Being overweight AND having a job that is strenuous on the knees (construction/manual labor/heavy lifting) is a bad combo which can lead to joint deterioration and premature joint replacement as well.