Chickens die much like everything else dies, sometimes suddenly and sometimes from age or reproductive related problems. Yours have lived longer than most laying breeds do, so that's a good thing.
Normally, a person will kill them when they notice one is suffering/not eating/standing around/getting thin for no reason in order to give them a good death after a good life. If you have a neighbor who will do that for your birds, I'd urge you to let them do so to the one that is not currently wanting to eat. Dying naturally, though natural, isn't too pleasant a way to go most of the time, so if you can give her mercy it's a good thing to do.
In the future, though there are always exceptions to the rule, note that a hen is on the time clock to dying once she stops laying her regular laying cycles she normally does in a year. People who want to avoid older hens suffering prior to death will normally kill them when they stop normal laying patterns, be they young or old, while they are still feeling good and having a good life. This serves three purposes~it gives a chicken a good life and a quick and merciful end to it, her meat can still be consumed, and it removes a potential carrier of parasites and/or disease from the flock.
Excellent! I'd give you an ovation if someone will tell me how!