I had parents who would tell me, patronizingly, that they were automatically smarter than me about things simply because they were older. This could easily hold true in many situations, but didn’t hold true in all of them, especially those which were most important to me. For instance, I had my father, an alcoholic, tell me he was smarter than I was about drinking, and knew what he was doing, and who was I to say he was imbibing too much?
As far as medical consent goes, I’m not sure how it is in the UK or the rest of europe, but in Canada you can appoint someone as your legal “agent” to make decisions for you in the event that you cannot make decisions for yourself. This doesn’t even necessarily need to be legally binding, however, in a situation where a family fight might ensue, it comes in handy if it is.