Chickens from good genetic stock will be healthier than those from weaker genetics. If she's a hatchery bird, then those in general tend to have more problems as an overall percentage. Still, you can get some good, long lived birds from them. At 6 years old then reproductive problems are not...
20 years and no messy butt's! You are lucky!
How old is the hen? Messy butts can have many causes, some birds just don't push droppings out as hard as others. Older, heavier birds seem to be more prone. Reproductive problems can make it harder for them to push droppings out and cause messy...