Really? I thought I'd heard that O Sensei met an older man who had never done a martial art, and personally trained him, that he enjoyed the children's classes, and encouraged Aikido as an art for everyone
I spent an hour as uke during open practice for a partner needing to work on shihonage. Another open practice I uke'd for an hour in koshinages. Neither time did I come away with sore joints. I think joints get injured when people either don't know the right way to move (eg planting the foot when turning, causing a lot of the knee injuries), or resist past the point of intelligent ukemi (ha...you can't make the technique work). One other rare cause is a nage (worst when they are large) who might muscle a pin once uke has submitted...but that is a serious error on nage's part and I don't consider it to be the price of training.
I'd say that anyone who comes away from training with sore joints is doing something wrong. Again, pain is not an indication of gain, it is an indication of injury.