I understand where you're coming from. I'm 45. I've had knee problems and back injuries in the past.
The knee problems pretty much mean you can't kick, which is what you indicated. You can maybe do low height stomping kicks or anything that doesn't involve "snapping" your kick out really fast like karate or taekwondo would have you do.
There are a number of martial arts that involve no snapping kicks and only do low angle kicks. Classical jujitsu is one example. They do kicks and punches, but there's no "snap" to them, and their kicks are typically low or mid-section only. Instead of using a snap, they create power by accelerating the kick or punch gradually outward, continuously through the entire motion, fully connected through their whole body to the ground. There are many jujitsu styles out there you could look at.
Back pain is the next variable. I don't know enough about the nature of your back pain to know what would work for you. I think that's just something you're going to have to do some trial and error on. Try a bunch of different martial arts and see whether your back is still okay at the end of the day.
You can try stick fighting (kali / escrima). That's a lot of fun and fairly practical. What you learn with sticks applies pretty much equally to empty hand, knife, and machete.
I could literally list off dozens of martial arts at this point. I think in the end, you're just going to have to do a little research, figure out what styles are being taught near you, show up at those places, and observe some classes. Ask the instructors when you get there whether your injuries would preclude you from participating.
Hope that helps.