The color indicates the age bracket of the patient, but the age bracket is already in the x axis. For example, what does the gray bar in the bracket 40-49 mean? Should be over 65 according to legend.
I agree the presentation is confusing. There are 4 categories, two are the same category except separated by age. The other two categories are not separated by age. There are two gray colors; the darker one is "NHAP patients, not bedridden" (nursing home acquired pneumonia), and is the one you refer to.
You'll note that only 3 categories are present at any age range except 60-69, because that decade includes both above/below 65.