It really depends on where you live. I don't know how counties outside the US operate, and in the US, each state has different policies regarding DNRs. You should be able to get some form of advance directives no matter your age, but that doesn't guarantee that everyone will just let you die. Some areas have very specific requirements on when to honor a DNR, so you should research the laws of your area. Also in some areas, a DNR can be invalidated if the form is damaged, so a disgruntled family member could just rip the paper and EMS workers would be forced to do CPR.