A new ladder is relatively inexpensive, you can at least experiment with them before you buy them.
I'm a shortarse, and I didn't realise it until after I had gone WFP, and then spent a day up a ladder helping out another window cleaner who'd had a hernia op and couldn't do his ladder work.
I've had both my knees key-holed and serviced

(a shame they can't fit a grease nipple, would make things so much easier

) and always assumed the damage was through a lifetime of sport and being overweight.
But after climbing the ladder for just a few hours, over the next few days I was really crocked again

So for me it was the cumulative effect of a lifetime up a ladder that trashed my knees.
And it is that resting them against the rungs that did the damage...
Would I also recommend knee pads?
Yep.
Also get the ladders with the angled rungs so that when you are stood on them, you are stood on the flat of the rung and not on the corner of the square rung ladders.
Ian