With respect, the author said his doctor told him not to. I am certain that the author and their medical professional know a hell of a lot more about what's a good idea for his body than you or I do.
replyWhen you are young and healthy, it feels like your body has no real hard limits, and doesn't define the boundary of what is and what isn't possible. But at some point, through age or misfortune, you will learn that, no, sometimes your body tells you "no" and you must listen.
There are amputees that do snowboarding with specially designed prosthesis and boards, so there is certainly a way to take load off weak knees with appropriate gear. OP is just, quite reasonably, not prioritizing this minor dream enough to invest so much time and money in it at the expense of other priorities.
replyA doctor knows about enough dangers and germs to never let you out the door!
replyI don't think the "is he physically capable or not" question is really relevant to the point the author is trying to make.
replyThe point is that you only have so much time, and you will never do all the things you want to do, and learning to deal with that fact is an important thing you have to do.