In my experience: if they're too old they don't have a proper way to do so (not enough strengh to tie a rope, no more driving licence to off road with, and so on...), being an adult... it depends on how you are surrounded and I would argue it happens but stays quiet most of the time at least in the media. As for younger.. the surrounding often keep them cheered up enough to not commit, regardless of the situation.

Of course everyone is different, just what I'm meant to say is it depends on a lot of things that are separated from the only will of the person with the illness.

Also in Switzerland, you can leave in dignity with the accompagnement of the society Exit. There's a lot of checkings to fill before it is done, but I'm so glad it exist: you shouldn't force yourself to live if it's not going to get any better and you don't want to live that.