My cat is losing hair, but is mostly healthy?

She is an 18 year old cat who is starting to lose memory, and somewhat weight. She is otherwise healthy, but is starting to lose the hair on her underbelly and around her knees. It isn't that bad to her, and what I mean by that is it isn't itchy or sore, but is kinda red in some places, and at first was dotted with small scabs in a few spots, but not anymore. But like I said, she isn't bothered by it, she is fine with it being rubbed, and doesn't scratch it. What is possibly wrong with her, or is it just aging, which I know it probably isn't.