>Our one cat had an operation on his leg so we put a little box as a stepping stone so that he could get onto the bed, and then onto the window sill. Turns out he didn't need it at all and could jump perfectly fine, but would still use the box all the time.

The problem is cats are really good at hiding pain - being physically able to jump doesn't mean it isn't uncomfortable or painful for him.

We have a pair of very senior cats (18/19ish years old) and make sure the house has little staircases dotted around to make getting on to their regular lounging spots easier, as we noticed they prefer to use a step if something happened to be available - while they are still fairly mobile, I suspect they are developing some level of arthritis. Given they've been good companions for nearly two decades it's the least we can do to make sure they're comfortable.