Some do. Quite a lot get some brown marks. Yet others hardly anything. A lot depends on any medical condition. My Frodo and Hebe, siblings have
different health problems. Both fifteen and Hebe's eyes are perfect. She does have sore mouth problems at times. Frodo's eyes have over the years got a lot of brown marks. He has CKD. It might be worth getting it checked out. Give you peace of mind rather than worrying about it. Blood pressure, diabetes etc can all change the eyes. But also old age, which has no cure. And is perfectly natural. I find the best way of noticing any changes is looking at, and comparing photographs of taken a while back, and up to date.
Julie.