Seven years old is old for chickens. While it is possible they can live several years longer and stay in good health (some do) the older they get the more likely they are to develop problems. I'm going through that now with my 14 year old medium sized dogs. They've both had serious medical issues. We haven't needed to put them down yet because of quality of life issues but both have been close. I'm not suggesting you put your hens down, not at all. I haven't put my dogs down yet. But at 7 years old I'm not surprised that they are having problems.
Not necessarily. There are two types of black sex links and red sex links. There are the commercial egg laying hybrids, developed to produce a Grade A Large sized eggs and to lay a lot of them. They are generally fairly small, about the size of a leghorn, so more of what they eat can go into egg production instead of them having to use more of it to maintain a large body. They are bred for one or two good production seasons and are then replaced because flockwide production drops. In their own way they are as specialized in egg production as Cornish Cross are in meat production. Because their system is so specialized they can develop medical issues. Not all do, especially of you don't overfeed them, but they do tend to develop problems.
The other type of black and red sex links sold by hatcheries are simply crosses of dual purpose breeds. They may get a small hybrid vigor boost but essentially they are like their parents, dual purpose chickens. They have larger bodies and may lay large or medium sized eggs, like their parents. Since their parents are "hatchery" chickens they tend to lay a lot of eggs but they are no more susceptible to egg laying problems than their parents are. Most hatchery BSL made from dual purpose breeds use a RIR or New Hampshire rooster over a Barred Rock hen, but some may use a Cuckoo Marans hen. There is more variety with the dual purpose RSL. The rooster is usually a RIR or New Hampshire though there may be a few others used but the hens can be White Rock, Silver Laced Wyandotte, Rhode Island White, Delaware, Light Sussex, or other breeds with the Silver gene.
OwO I don't know which type of BSL your hen is. But either way for her to still be laying well at that age you are doing something right.
The color/pattern for Light Sussex is a very common pattern across many breeds or even mixes. Body shape and configuration has more to do with breed than feather color or pattern. I don't know where you got those or who the breeder was but with a different body shape they are not going to win any chicken shows if entered as Light Sussex. Whatever they are, they are still chickens.
You are taking her to a vet that knows chickens. Hopefully they can figure out what is going on. If it is EYP (it may not be) that means the yolk is going in her body cavity when it is released instead of through her internal egg making factory. Sometimes a hen just has an "oops". Something just doesn't go right. That's why you can sometimes get strange eggs. In most cases if it is a rare oops, well I figure we are all entitled to an occasional oops. It's when it becomes consistent that it shows something is wrong. Sometimes if it is consistent that can be treated, sometimes it means the hen is defective. Sometimes an occasional oops can threaten their health. This may be one of those cases. I hate to mention this but you are going to a vet. Certain diseases can cause egg laying problems. Hopefully that can be eliminated.
I can't see that you are doing anything wrong. With 7-year-old hens medical problems are sort of to be expected. To get them this far in that good of shape you have done well. A for the younger one, it could just be rotten luck. That happens sometimes. It's a downside of animal husbandry.
Good luck!
