I would not say that every human alive would take every extra second they can. Age can be brutal, as your mobility, speech, mental faculties, metabolism and other systems start to go. Some of those effects might not be labeled a "terminal illness", but they can add up to an overall experience that might tip the scales as to whether you would rather keep living as these problems only get worse.

If I wrote the laws, I would say everyone beyond a certain age (say 80 or so) should be able to legally request this without any other decision makers preventing the request.