I'm not familiar with ISA browns, but any hen that has lived long enough to be finished laying is going to be a tough bird. They all pretty much become tougher the older they get. Older than 14 weeks you're no longer looking at a fryer. A slow roaster, maybe, until about 20 weeks.
Layers, even the quickest maturing, don't even start to lay until at least 20 weeks.
After that, though some will disagree, they're pretty much best cooked in the crock pot, IMO. But, older the bird is, the more flavor it has.
Coq au Vin is traditionally made with an older rooster, because they have more flavor. Get some recipes for slow-cooking chicken, if you plan to eat spent layers. They make yummy chicken 'n dumplings, but also much more.
Hens can lay for years. I have had hens lay for at least 6 years. Production wanes after the 2nd year, (hens lay their largest eggs from the 2nd year on) but depending on what your needs are, you may not want to replace all your layers every two years. If you don't require a hen lay every single day, you might be happy with 3 and 4 year old hens that lay 4 or 5 eggs a week.