It's funny that prunes are somehow equated with "elderly people". There was even a Star Trek episode where the warrior-Klingon Worf was enamored with prune juice, and it was a joke because to humans this is a drink of wimps.
Prunes are simply dried plums, just like raisins are dried up grapes!
Prunes and plums are both high in antioxidants, which are important in maintaining your body's health and fighting off cancer. These are the same components that make red wine so powerful.
Fruits have naturally occuring sugars in them, which give them their sweet flavor. If you have a serving of five prunes, you get about 13g of sugar and 15g of sorbitol - i.e. sugar alcohol. There's also 3g of fiber. The sorbitol is not usually counted in net carbs, but the sugar of course is :)
Thirteen grams of sugar in one small snack portion is rather high for someone trying to lose weight, but it would be fine for someone on maintenance who wants a sweet snack for a special occasion. They get sugar, but they get a lot of nutrition and antioxidant with that sugar. It's far better than eating candy!
Lisa Shea's Library of Low Carb Books