Old(ish??) hens prone to illness?

[WannabeHomsteader]

I have 2 laying hens and 3 pullets. The layers are ISA browns, both a little over 2 years old and still laying every other day or so. I always feel like they're struggling with issues - poor feather growth, unresolved respiratory issues, just weird stuff I can't find causes or solutions for. Furthermore, they're problems that my pullets don't deal with yet, that I have seen. I have reviewed my management practices, space requirements, cleanliness and biosecurity... I don't claim to be the perfect chicken keeper by any stretch, but I am doing my best.

To be clear, I am not looking for treatment plans or suggestions on making them better. Here are my questions I have...
- I know since they're production birds, their lifespan will probably be shorter than average. But do they age faster overall? Or is it only their reproductive system that goes kaput, after laying intensively? I'm trying so hard to figure out why they're my "problem birds".
- When you end up with birds that struggle with their health, when do you decide culling is the better option? How do you evaluate pros and cons? If it helps, these hens are like not-quite-pets for me. I definitely enjoy them, but I don't feel the same way as I would with a dog or a companion animal. And the constant issue management is starting to take the enjoyment out of chicken keeping for me.