I don't think you understood the point. If it really is your "dream" you adapt, as someone else pointed out, there's people without knees who snowboard.... there's a bunch of things you can do. But quite clearly, he hadn't even started on that journey, he didn't even know if he would have enjoyed it. it was a fantasy. Trust me, I get it, I have bad knees, but during my 40s my fantasy was to do parkour, and I did, I just adapted and got pretty good at it, now in my 50s, I don't do parkour anymore, but have a bunch of other problems and I still work out how to do the things I want.