My advice for anyone 18 or over, but mainly for middle aged, people who are seriously ill or older people. Get an Advance Directive in place. You can download templates online. It's not just for elderly people. Think about what you'd want if you had a bad accident, a stroke, a neurodegenerative illness, serious car crash, dementia etc. This advice is for everyone - pro and against AD, religious and non religious, anyone and everyone.
Things to consider:
Resuscitation in the event of a cardiac arrest in which you'd be unlikely to recover reasonable function/self care abilities.
ITU and intubation/treatment in the event of a catastrophic accident which would leave you in effect minimally conscious (vegetated) or with severe disability.
Treatment you'd want if you suffered a severe stroke/subarachnoid haemorrhage if you'd be unlikely to recover reasonable ability/self care abilities.
Artificial feeding via PEG (tube directly into stomach), TPN (via blood vessel) or supplementary feeding if you couldn't swallow very well.
Repeated courses of antibiotics for chest infection.
Pacemaker for heart arrhythmia/disease.
Life saving surgical procedures if you had a poor quality of life/poor self care/very advanced age.
A description of how you want to life your life/what you want to be able to do/what you are not prepared to accept etc. Everyone will be different.
These issues tend to be more urgent as you age. Talk to your next of kin and family members about what you want as you age/fall ill/become disabled or less able. If you have an Advance Directive your family can't over rule this. You should appoint Power of Attorney for health if you can, but still put an AD in place as families can and do over rule people's wishes, especially if doctors don't know what they are. I nursed someone who had refused placement of a PEG tube, but her daughter insisted and she lived for the last years of her life paralysed, unable to speak or do anything for herself although she was conscious.
Get it signed, dated and witnessed by an independent person, a neighbour or friend will do. Lodge it with your GP surgery and check they've included it on your health record. Re-sign every two years. Keep a copy for yourself.
Prolonging your life in a miserable and painful state unable to do your self care abilities and not wanting to be here can be avoided.