Iím 49 and have a bad back. What is a comfortable kayak that I can fish with and not kill my back.
I've had a bad back for years and I'm 72.
I'd suggest that you use a foam cushion no matter how comfortable a seat may seem.
The newer kayaks come with the reclining lawn chair type seats that adjust in height that allow you to have some elevation to your legs, that relieves having to sit straight legged in the cockpit which to me always put more pressure on the hips and back.
Where you will really struggle, and it's something that a lot of people with back problems fail to even consider, and that is loading and unloading, as many of those same newer kayaks are pushing 80, 90, to 100# and a lot of them are only 10' kayaks so they are a brick.
A trailer is going to be the way to go if you choose a kayak over around 65# which even then will feel heavy after a few hours of fishing and paddling.
My present kayak weighs around 60# if I remove the seat, and everything that will come out of it.
You could look into a sit inside type kayak if hauling would be a concern, and they can weigh in at around 45 to 55 pounds. I've fished out of sit inside kayaks, and they are stable and paddle with ease, usually they have a fairly comfortable seat, but can be a little hot in the summer unless it's more of an open cockpit model.
You can get a sit on top that weighs about the same, but you will not have an elevated seat and it will be hard on the butt and back with the seat that come with them, and they may not be back friendly.
Next time you page through one of those magazines that show the different models of kayaks, take a look at the weight specifications and you will be able to narrow them down fairly easy.
Remember, you are out to enjoy the sport, and you don't want to dread having to load and unload your new kayak, and possibly make your condition worse.