It's not because of illnes, but everybody will be more skinny, when will be more older later. Our skin is lost the water, when people are older, that's why our skin is follow our bones' form, and most of us will be more skinny. Of course, it's depend from a lot of thing, as the person's life, environment, and the genetics too (For example, my grand-grandmother looks like when she was 80 years old, like a 60-70 years old, and doesn't was skinny). It's just a little tip, when you draw an older person in your picture/story.