Are these the only chickens you have?
If you have others, please isolate these chickens so that your others do not catch what they have.
There are so many things that this could be. Chickens look fine right up until they keel over. I guess it is an evolutionary advantage: if they appeared to be sick, predators would notice and eat them.
That said, it sounds like you got some bum chickens. They could be any age, and they could have any number of illnesses.
What are you feeding them? Are you free-feeding them dry layer pellets and oyster shell? Are you keeping them out of areas that have decomposing matter that might give them a fungal disease or botulism poisoning? Is their water changed daily so that they do not contact a fungal disease? Do they have worms? Have you checked them over carefully for lice, mites, injuries?
If you have time, you might want to call your State Extension Office and see if you can get a free necropsy to make sure there were no serious diseases involved.
Sorry about your chicken.